WARSAW—For years, thousands of U.S. and NATO troops have stood guard in Poland—a presence designed to deter an invasion by Russian troops and tanks.

This week, the Western alliance faced a less conventional challenge: At least 2,000 people from the Middle East trying to cross into Poland from Russia’s closest ally, Belarus.

European officials...

WARSAW—For years, thousands of U.S. and NATO troops have stood guard in Poland—a presence designed to deter an invasion by Russian troops and tanks.

This week, the Western alliance faced a less conventional challenge: At least 2,000 people from the Middle East trying to cross into Poland from Russia’s closest ally, Belarus.

European officials accuse Belarus of abetting human traffickers bringing migrants into their country and then of funneling the new arrivals toward the border in an effort to provoke a crisis—things Belarus denies.

Tensions are rising. Poland has deployed more soldiers along the frontier to keep the would-be crossers out. Russia says it views the troop movements as a threat and has responded by sending bombers to patrol over Belarus.

Alexander Lukashenko, Belarus’s authoritarian president, noted in a press conference that the Russian warplanes were capable of carrying nuclear weapons.

Police near Hajnowka in eastern Poland on Thursday.

Photo: wojtek radwanski/Agence France-Presse/Getty Images

“What we are dealing with is a new type of war,” Polish Prime Minister Mateusz Morawiecki said in a Facebook post Thursday. “This is a war in which civilians and media messages are the ammunition.”

Poland’s government says Belarus is engaged in an “act of hybrid aggression” to provoke a clash at the borders—playing out in full view of the world. Other Western officials say they think Russian President Vladimir Putin wants to create a dramatic spectacle and undermine the West, but stop far short of actual armed conflict.

The defense ministers of Estonia, Latvia and Lithuania warned Thursday of the risk of military confrontation. Russia, which has denied any involvement, said the European Union should speak with Belarus to resolve the crisis.

Belarusian Defense Minister Viktor Khrenin said its neighbors’ military activities—particularly in Poland—are unrelated to the migrant crisis and may signal they “are ready to unleash a conflict in which they want to involve Europe in solving their internal political problems, as well as problems related to relations within the European Union.”

The standoff involves far fewer migrants than the flows that poured into the continent after 2015. On most days, only a few dozen asylum seekers are managing to get across the border. But the imagery of cold and hungry migrants, directed by Belarusian troops toward a single border checkpoint, has played out on social media for days, roiling European politics.

Belarus on Thursday allowed staff from the United Nations’ refugee agency and International Organization for Migration to visit a makeshift camp near the border housing roughly 2,000 people, including children and women, many pregnant, the agency said Friday. Aid workers are racing to provide assistance as winter approaches.

“The makeshift camp at the border with no adequate shelter, food, water and medical care in freezing temperatures is not a safe and suitable place for people and could lead to further loss of life,” the U.N. agency said in a statement.

Several refuge-seekers have died at the border since the crisis began this summer. Belarus’s border agency said Friday that a migrant teenager was treated for hypothermia overnight by Belarusian doctors and later returned to his family.

Since summer, Belarus has issued tourist visas to people from Iraq, Syria, Yemen and other countries in what EU officials say is an orchestrated effort to get them to the EU’s borders.

Thousands of migrants massed at the eastern border of the European Union, some cutting razor wire and attempting to climb over fences between Belarus and Poland. Polish troops were mobilized to prevent them from crossing. Photo: Leonid Shcheglov/Zuma Press The Wall Street Journal Interactive Edition

European officials have been pressing airlines across the Middle East and Turkey to significantly limit the number of passengers they carry to Minsk, winning some promises of cooperation.

Belarus and Russia are unlikely to risk a military clash against the North Atlantic Treaty Organization, officials and analysts say. Instead, Moscow and its allies are seeking to sow dissension. In recent years, tactics have ranged from disinformation to holding back gas supplies while energy prices in Europe soar. At a government meeting Thursday, Mr. Lukashenko floated the idea of halting gas transit from Russia to the EU. The Kremlin said Friday that wouldn’t happen.

For Europe, the big fear is a repeat of 2015, when more than a million asylum seekers crossed into the continent, creating scenes that fed a surge in popularity for nationalist parties and helped nudge the U.K. out of the EU. The numbers camped in Belarus now are comparably small: a few thousand.

“This is as much a PR campaign as a breach-of-the-border campaign,” said Elisabeth Braw, a senior fellow at the American Enterprise Institute, a Washington-based think tank. “They know how divisive immigration is and how these images will create a toxic divide between citizens and different countries’ governments.”

Polish troops standing guard Thursday at the border near Kuznica.

Photo: HANDOUT/via REUTERS

For Poland, and for neighboring Latvia and Lithuania, the specific fear is a crisis that escalates to a stage where larger powers—Russia, Germany, or the EU—step in to resolve it.

All three EU nations that neighbor Belarus have blocked journalists, activists, and nongovernmental organizations from approaching the border, with police establishing checkpoints along the roads leading to the area. Their governments have declared states of emergency in their border regions, curtailing ordinary democratic freedoms to manage what they see as a threat to their sovereignty.

Since Monday, the Polish government has discussed triggering Article 4 of NATO’s founding treaty, which would launch consultations among the military alliance’s members about a security threat. The article has been triggered several times, including by Poland in 2014 over Russia’s invasion of Ukraine and by Turkey in 2020 over the situation in Syria.

Poland has also weighed various offers of help from foreign capitals but has largely rebuffed them. The government has effectively sidelined Frontex, the Warsaw-based EU border-control agency. Troops from NATO allies based only a few hours from the border region have stayed back.

“We would like to avoid a situation in which Brussels or Berlin would over our head reach some kind of agreement,” said a Polish government official, referring to the political capitals of the EU and Germany. “We can see some analysis where Russian border guards will in a week or two somehow find themselves near the Belarus border: ‘Oh, Belarus is not able to handle their situation, but we can come and handle it for them.’”

“This is our business and we need to be a little bit overprotective about it,” he said.

EU leaders have called for sanctions on Belarus and airlines that are flying migrants to the country. NATO allies expressed solidarity with Poland and its neighbors after a meeting Wednesday. But the alliance has taken no practical steps to reinforce the region. NATO battle groups there are fully operational, but they are not involved in the crisis and their alert state remains unchanged, a NATO official said.

Belarusian President Alexander Lukashenko, center, this week floated the idea of halting gas transit from Russia to the EU.

Photo: Nikolay Petrov/Associated Press

Russia has sent bombers to patrol over Belarus, a close ally.

Photo: /Associated Press

On Friday, the British military said it would send a small team “to explore how we can provide engineering support to address the ongoing situation at the Belarus border.”

At the same time, Russia is maneuvering some military forces near Ukraine, sparking concerns in Washington that it may be planning to try to expand its military interventions on its neighbor’s territory. Russia, which wants to draw Ukraine back into its sphere of influence, launched covert invasions of Crimea and eastern Ukraine in 2014 after amassing troops on the border.

Ukrainian officials say the buildup of troops is less menacing than in spring this year, as the units are less numerous and further away, but that Moscow’s intentions are unclear. A Kremlin spokesman said the Russian troops were moving on their national territory and don’t pose a threat to anyone.

The U.S. is concerned that Russia “may make the serious mistake of attempting to rehash what it undertook back in 2014,” Secretary of State Antony Blinken said Wednesday after meeting his Ukrainian counterpart.

“The playbook that we’ve seen in the past is to claim some provocation as a rationale for doing what it’s intended and planned to do all along, which is why we’re looking at this very carefully,” Mr. Blinken said.

Write to James Marson at james.marson@wsj.com and Drew Hinshaw at drew.hinshaw@wsj.com