The European Union accused Belarus on Wednesday of mounting a "hybrid attack" by pushing migrants across the border into Poland, paving the way for new sanctions against Minsk in a crisis that threatens to draw in Russia and NATO.

Russia took the rare step of dispatching two nuclear-capable strategic bombers to patrol Belarusian airspace in a show of support for its close ally. NATO signalled its backing for member state Poland.

Migrants from the Middle East, Afghanistan and Africa trapped in Belarus made multiple attempts to force their way into Poland overnight, Warsaw said, announcing that it had reinforced the border with extra guards.

UN human rights chief Michelle Bachelet called on states to deescalate and resolve the "intolerable" crisis.

"These hundreds of men, women and children must not be forced to spend another night in freezing weather without adequate shelter, food, water and medical care," she said.

The EU, which has repeatedly sanctioned Belarus for human rights abuses, accuses Minsk of drawing in migrants from war-torn and impoverished countries and then pushing them to cross into Poland to try to sow violent chaos on the bloc's eastern flank.

"We are facing a brutal hybrid attack on our EU borders. Belarus is weaponizing migrants' distress in a cynical and shocking way," EU Council President Charles Michel said.

The bloc's 27 ambassadors agreed this constituted a legal basis for further sanctions, which could come as early as next week and target some 30 individuals and entitities including the Belarusian foreign minister and the national airline. 

Belarusian President Alexander Lukashenko and his ally, Russia's Vladimir Putin, have placed the blame on the EU.

The Kremlin accused Europe of failing to live up to its own humanitarian ideals and trying to "strangle" Belarus with plans to close part of the frontier. Moscow said it was unacceptable for the EU to impose sanctions on Belarus over the crisis.

PRESSURE POINT

The crisis strikes the EU in a vulnerable area.

In 2015, the bloc was deeply shaken by an influx of more than a million people fleeing conflict in Syria, Iraq and Afghanistan that led to deep rifts between member states, strained social security systems and fanned support for far-right parties.

The EU appears more united this time but there are signs of internal friction: some in Brussels have warned Poland that it should not use EU funds to erect border walls and razor wire while others argue the bloc needs to help defend its borders. Michel said on Wednesday the EU needed to make up its mind.

Compared with 2015, the current crisis has an added geopolitical dimension as it is unfolding on the dividing line between NATO to the west and Russia-allied Belarus to the east.

The Tupolev Tu-22M3 bombers that Russia sent to overfly Belarus are capable of carrying nuclear missiles, including hypersonic ones of the kind designed to evade sophisticated Western air defences.

Russian Foreign Minister Sergei Lavrov said he hoped responsible Europeans would "not allow themselves to be drawn into a spiral that is fairly dangerous".

Poland briefed its NATO allies on Wednesday in Brussels.

"Allies made clear that Belarus bears responsibility for the crisis and that the Lukashenko regime's use of migrants as a hybrid tactic is inhumane, illegal and unacceptable," a NATO official said after the closed-door meeting.

"NATO stands ready to further assist our allies, and maintain safety and security in the region," the official said.

German Chancellor Angela Merkel urged Putin to put pressure on Belarus, a German government spokesperson said. The Kremlin said Putin told her the EU should talk directly to Belarus.

STRANDED FAMILIES

Thousands of people have converged on the border this week, where makeshift razor wire fences and Polish soldiers have repeatedly blocked their entry. Some of the migrants have used logs, spades and other implements to try to break through.

"It was not a calm night. Indeed, there were many attempts to breach the Polish border," Polish Defence Minister Mariusz Blaszczak told broadcaster PR1.

Video from the border obtained by Reuters showed young children and babies among the people stuck there.

"There are lots of families here with babies between two and four months old. They have not eaten anything for the past three days," the person who provided the video told Reuters, saying they were a migrant themselves and declining to be identified.

Some migrants have complained of being repeatedly pushed back and forth by Polish and Belarusian border guards, putting them at risk of hypothermia, lack of food and water.

Reuters found ripped up tickets from Middle Eastern airlines, documents from tourist agencies and receipts in the forest near the Polish town of Hajnowka at what appeared to be an abandoned camp site.

Poland's prime minister said the EU needed to block flights from the Middle East to Belarus.

Poland denies accusations by humanitarian groups that it is violating the international right to asylum by hustling migrants back into Belarus instead of accepting their applications for protection. Warsaw says its actions are legal.