Poland says it has repelled an attempt by hundreds of migrants and refugees to enter the country from its eastern neighbour, Belarus, and said thousands of others were on the way.

Warsaw said it had deployed additional soldiers, border guards and police in response to the situation along its eastern border, while video clips shared on social media purportedly showed people walking towards the frontier and some trying to breach a barbed-wire border fence using spades and other items.

The developments appeared to signal an escalation of a crisis that has been rumbling for months.

The European Union has accused Minsk of encouraging migrants from the Middle East and Africa to cross into the bloc via member states Poland, Lithuania and Latvia from Belarus as a form of hybrid warfare in revenge for Western sanctions on longtime President Alexander Lukashenko’s government.

Lukashenko, who launched a crackdown on dissent in Belarus after winning a sixth term in a disputed presidential election held in August last year, has repeatedly denied those accusations.

Warsaw blocks ‘mass attempt’

Poland said it had withstood the first attempts on Monday by those gathered along its border to force their way across the frontier.

A video distributed by Polish authorities showed one man cutting part of a barbed-wire fence, another attacking the fence with a spade, while a Polish soldier sprayed an unidentified substance from a can.

"Interior ministry forces and soldiers managed to stop the first mass attempt to breach the border,” Poland’s defence ministry tweeted.

"Migrants have set up a camp in the Kuznica region. They are constantly guarded by Belarusian services.”

In an earlier video, shared by the Belarusian blogging service NEXTA, people carrying rucksacks and wearing winter clothing were seen walking on the side of a highway. Other videos showed large groups sitting by the road and being escorted by armed men dressed in khaki.

Polish government spokesman Piotr Muller told reporters that 3,000-4,000 migrants were near the Polish border.

"We expect that there may be an escalation of this type of action on the Polish border in the near future, which will be of an armed nature,” he added.

The Belarusian state border committee confirmed on Monday that many migrants and refugees were moving towards the Polish border. It also said Warsaw, which has stationed more than 12,000 troops in the region, was taking an "inhumane attitude”.

Lithuania, meanwhile, said it was moving additional troops to its border with Belarus to prepare for a possible surge in attempted crossings from the latter, while Latvia described the situation as "alarming”.

EU calls for more sanctions

The EU vowed action in the face of what it said was a "hybrid threat”.

In a statement later on Monday, European Commission President Ursula von der Leyen called on member states to impose new sanctions against Belarus.

The use "of migrants for political purposes is unacceptable”, she said in a statement. Brussels would also look at how to sanction "third-country airlines” that brought migrants to Belarus, she added.

A statement from the US State Department called on the government in Belarus to "immediately halt its campaign of orchestrating and coercing irregular migrant flows across its borders into Europe”.

Al Jazeera’s Dominic Kane, reporting from the German city of Frankfurt an der Oder, on the country’s border with Poland, said the developments at the Polish-Belarus border were of significance for the EU on two fronts.

"It represents the two border issues with which the EU has to wrestle – the first one, its inner borders within member states, and the second one its outer borders, between member states and non-EU countries,” Kane said.

"The EU wants to keep its borders within its member states wide open, but some of its borders, particularly with Belarus, very much more firmly under control,” he added.

Russia backs Minsk

Exiled Belarusian opposition leader Sviatlana Tsikhanouskaya, meanwhile, urged a strong response from the EU and United Nations.

"Belarus’ regime escalates the border crisis – migrants are pushed to EU border by armed men,” she tweeted. "The migrant smuggling, violence & ill-treatment must stop.”

The EU, the United States and United Kingdom all imposed sanctions on Belarus after Lukashenko unleashed a violent crackdown on mass protests following last year’s presidential vote. All of Belarus’s top opposition leaders are now either in prison or have fled the country.

But the 67-year-old, who has accused Western governments of having instigated the protests against him in the hope of fomenting a revolution, has so far withstood the penalties with the backing of ally and creditor Russia.

Kremlin spokesman Dmitry Peskov on Monday defended Minsk’s handling of the border crisis, saying Belarus was taking all necessary measures to act legally.

But human rights groups have criticised both Poland and Belarus for their treatment of migrants and refugees, who face freezing weather conditions and a lack of food and medical attention.

Warsaw has said at least seven people have died in the region since August, when the border crisis erupted. There have also been reports of more deaths on the Belarusian side of the frontier.