In a grim backdrop to intense diplomacy aimed at preventing war between Russia and Ukraine, thousands of troops sent by Moscow to Belarus engaged in military drills. Convoys of Russian anti-aircraft missile systems rumbled along snow-covered roads as part of the maneuvers.

Meanwhile, more NATO forces also are on the move, heading to the alliance's eastern borders, while Britain is putting 1,000 troops on standby to respond to a possible humanitarian crisis in Eastern Europe i f Russia invades neighboring Ukraine and fighting breaks out.

Russia has concentrated more than 100,000 troops near Ukraine's border, but says it has no invasion planned. It wants guarantees from the West that NATO won't allow Ukraine and other former Soviet nations to join the Western military alliance.

Foreign policy advisers from Germany, France, Russia and Ukraine, who met in Paris last month, held another round of talks in Berlin. They reported no progress on the implementation of a 2015 peace agreement that helped end full-scale hostilities between Ukrainian forces aUkraine, d Russia-backed separatists in eastern Ukraine.

Here's a look at what is happening where and why:

Biden Urges Americans To Leave Ukraine

U.S. President Joe Biden has repeated his warning that any Americans still in Ukraine should leave as soon as possible.

“It's not like we're dealing with a terrorist organization. We're dealing with one of the largest armies in the world. It's a very different situation and things could go crazy quickly,” he said in an interview with NBC News broadcast Thursday.

?Asked whether there was any scenario that would prompt him to send U.S. troops to Ukraine to rescue Americans, the president said: “There's not. That's a world war when Americans and Russia start shooting at one another.”

He argued that if Putin is “foolish enough to go in, he's smart enough not to, in fact, do anything that would negatively impact on American citizens.”

Asked whether he's ever said that to Putin, Biden said he had.

“I didn't have to tell him that. I've spoken about that. He knows that,” Biden said.

The State Department for weeks has advised Americans in Ukraine to leave the country.

Boris Johnson At NATO, Then In Poland

British Prime Minister Boris Johnson and NATO Secretary General Jens Stoltenberg are warning that Russia's ongoing military buildup near Ukraine poses one of the biggest threats to European security in recent memory.

“This is probably the most dangerous moment, I would say in the course of the next few days, in what is the biggest security crisis that Europe has faced for decades, and we've got to get it right,” Johnson told reporters at NATO headquarters in Brussels.

The British leader said he doesn't believe that Russian President Vladimir Putin has decided to take action against Ukraine. But, Johnson said, “our intelligence, I'm afraid to say, remains grim.”

Stoltenberg warned: “The number of Russian forces is going up, the warning time for a possible attack is going down.”

He stressed that “NATO is not a threat to Russia, but we must be prepared for the worst.”

Stoltenberg said he sent a new letter to Russian Foreign Minister Sergey Lavrov repeating NATO's invitation to a series of talks on improving European security.

Johnson traveled from Brussels to Warsaw for talks with Polish Prime Minister Mateusz Morawiecki.

The two leaders went to meet British soldiers stationed near Poland's capital as part of a NATO mission to augment the alliance's eastern flank. Poland borders Belarus, Ukraine and Russia's Kaliningrad region.

“Putin's political goal is to break up NATO. That's why we need to stay close together," Morawiecki said.

“On one side of the scales, there is freedom and European security. On the other side, there is destabilization and the things that the Kremlin is cooking up," he said.

“We want to build bridges and connections with Russia, but we want to build them on the foundation of peace," the Polish prime minister said.

Truss And Lavrov Talks

British Foreign Secretary Liz Truss held talks with Lavrov in Moscow, with Truss again warning that attacking Ukraine would “have massive consequences and carry severe costs.”

Truss urged Russia to “abandon Cold War rhetoric” and choose the diplomatic route to avoid bloodshed in Ukraine.

Lavrov was in no mood to accept a lecture from the West during the first meeting in four years between Britain and Russia's top diplomats.

“Ideological approaches, ultimatums and moralizing is a road to nowhere,” he said.

Ukraine Accepts Turkey's Mediation Offer

Turkish Foreign Minister Mevlut Cavusoglu says Ukraine has accepted Ankara's offer of mediation to reduce the tensions with Russia, while Moscow has not ruled out the idea.

“Our discussions with both sides are continuing with sincerity,” Cavusoglu told state broadcaster TRT. “The Ukrainian side wants this. The Russian side has not closed the door, but we will still have contacts with them.”

Without naming any countries, Cavusoglu also criticized some Western diplomatic efforts, saying they were stoking tensions instead of easing them.

“Are there tensions? Yes. Are there risks? There are. Is the situation fragile? It is. There is concern, there is the risk of a possible conflict. But to announce this, like some Western countries are doing, through megaphone diplomacy is of no benefit,” Cavusoglu said.

British And U.S. Troops On The Move

Britain is putting 1,000 troops on standby to respond to any “humanitarian crisis” in Eastern Europe sparked by a Russian invasion of Ukraine. The U.K. fears a large-scale incursion could cause a mass displacement of people from Ukraine to neighboring countries.

The U.K. is also sending hundreds of troops to Estonia and Poland as part of a NATO show of strength.

Johnson is expected to offer to deploy more RAF jets to Southern Europe and to send Royal Navy vessels to the eastern Mediterranean.

The U.S. also is sending vessels to European waters. The Navy did not directly tie the deployment of four destroyers to the Ukraine crisis but said they will provide “additional flexibility” to the U.S. Sixth Fleet commander, whose area of responsibility includes the Mediterranean, and will operate in support of NATO allies.

The four ships are the USS Mitscher and USS Gonzalez, homeported at Norfolk, Virginia, and the USS The Sullivans and USS Donald Cook, based at Mayport, Florida.

Denmark Plans Defense Talks With US

Denmark said Thursday it will begin negotiations ?on a new defense cooperation agreement with the U.S. that may include allowing American troops and military equipment to be stationed on Danish soil — in reversal of a decades-old policy.

However, Danish Prime Minister Mette Frederiksen stressed that the move is not a response to current Russia-Ukraine tensions. She said talks on intensifying military cooperation between Copenhagen and Washington have been in the works for a long time.

“An increased American commitment here in Denmark will improve the access of the United States to the European continent,” Frederiksen said in comments quoted by Danish public broadcaster DR.

Danish Defense Minister Morten Boedskov told reporters that no American military bases would be established in Denmark, and he declined to comment on where U.S. troops would be placed in the country.?