The United Kingdom and Germany will increase their military deployment in eastern Europe amid the ongoing crisis between Russia and Ukraine.

U.K. Defence Secretary Ben Wallace on Monday said 350 more British troops would be sent to the Polish border to shore up NATO's eastern flank against any Russian aggression.

Wallace said the troops would strengthen a contingent of 100 British soldiers already there and would be a "bilateral deployment to show that we can work together and send a strong signal that Britain and Poland stand side by side."

The announcement came following talks between Wallace and Polish Defence Minister Mariusz Blaszczak amid growing fears of a Russian invasion of Ukraine, which could spark a massive exodus of refugees into Poland.

Belarus last year allowed migrants to cross its border into Poland, sparking a humanitarian crisis and prompting the deployment of 100 British troops to strengthen the border.

Blaszczak told journalists that the additional British troops would "cooperate with the Polish armed forces to increase security, especially when it comes to the threats on the border."

Wallace noted that "recently Belarus cynically used human beings as a way of destabilizing neighbors."

"Using illegal migration or indeed forcing people across the border, Britain stood shoulder-to-shoulder with Poland, and sent 100 Royal engineers to support them in those efforts to counter the use of migrants as a tool."

U.S. officials have said the Kremlin has assembled 110,000 troops along the border with pro-Western neighbor Ukraine but intelligence assessments have not determined whether President Vladimir Putin has actually decided to invade.

The officials added that an invasion could trigger a refugee flood of up to five million people, mainly into Poland.

Russia is seeking a guarantee from NATO that Ukraine will not enter the alliance and wants the bloc to withdraw forces from member states in eastern Europe.

Wallace on Monday insisted that NATO is a defensive alliance and that Russian aggression would be counterproductive and lead to "more NATO, the very thing that President Putin says he doesn't want."

He said that Russia's "2014 illegal annexation of Crimea led to an enhanced forward presence in Poland, Estonia, and Latvia."

Also, Germany will send up to 350 more soldiers to Lithuania to help bolster the NATO alliance's eastern flank, Defence Minister Christine Lambrecht said Monday, amid fears that Russia could invade Ukraine.

"We are therefore strengthening our troop contribution on NATO's eastern flank and sending a clear sign of our resolve to our allies," Lambrecht said, adding that the soldiers will be deployed "within a few days."

The message to allies is, "you can rely on us," said the minister.

Germany currently has 500 soldiers stationed as part of NATO forces in Lithuania.

The boosted German force in the Baltic nation would make Lithuania Germany's second-biggest deployment after Mali.

Berlin had come under fire over its refusal to send weapons to Ukraine despite repeated requests from Kyiv.

Its offer to instead dispatch 5,000 helmets was widely ridiculed recently.

With tens of thousands of Russian troops massed on the Ukrainian border, fears are mounting that a major conflict could break out in Europe.

An intense round of diplomacy is ongoing in a bid to stop the situation from spiraling out of control.

German Chancellor Olaf Scholz is in Washington for talks with U.S. President Joe Biden, while Foreign Minister Annalena Baerbock is in Kyiv to meet her Ukrainian counterpart.

Scholz said on Monday there would be a very high price if Russia invades Ukraine, amid U.S. warnings that a Russian invasion could happen in days or weeks.

Scholz was speaking to reporters before a meeting later in the day with U.S. President Joe Biden at the White House.

Germany's foreign minister has also urged Moscow on Monday to take steps to de-escalate tensions, and warned that Russia will face severe political and economic sanctions if it attacks Ukraine.

"We will respond with tough, very specific measures to any further aggression by Russia," Annalena Baerbock said in a statement.

She made the remarks ahead of her official talks in the Ukrainian capital of Kyiv, amid heightened tensions with Russia.

Baerbock reaffirmed Germany's strong support for the sovereignty and independence of Ukraine.

"As a partner of Ukraine in the EU, NATO and the G7, we categorically support the territorial integrity of the country and stand alongside the people," she stressed.

The top German diplomat is in Kyiv for her second visit within a month to demonstrate solidarity with Ukraine. She is also scheduled to travel to the "contact line" in eastern Ukraine to gain a first-hand impression of the situation.

Germany is among the largest donor countries to Ukraine and provided nearly €1.83 billion ($2.09 billion) in development aid since 2014, according to government figures.

But Germany's opposition to military support to Ukraine remains a source of friction and has been widely criticized by Ukrainian politicians.

Berlin has so far turned down Ukraine's requests for weapons pointing to the country's restrictive arms export policy, and arguing that this could further escalate military tensions in the region.