WASHINGTON: U.S. President Joe Biden cautioned American citizens about staying in Ukraineinfo-icon on Thursday, urging them to leave the country immediately and warning of a potential major conflictinfo-icon with Russiainfo-icon should a clash erupts between U.S. and Russian troops.

Speaking in a televised interviewinfo-icon on the U.S.-based television network NBC News, Biden said "American citizens should leave, should leave now."

"We're dealing with one of the largest armies in the worldinfo-icon. This is a very different situation, and things could go crazy quickly," Biden said.

The U.S. president said he would not send troops to Ukraine, even to rescue Americans in case of a Russian invasion.

"That's a world warinfo-icon. When Americans and Russians start shooting one another, we're in a very different world," he said.

U.S. Secretary of State Antony Blinken, speaking to reporters in Australiainfo-icon where he is meeting Friday with his counterparts from Australia, Japaninfo-icon and Indiainfo-icon, the so-called QUAD, reiterated Biden's warning, saying Russia could invade Ukraine during the Beijing Winter Olympicsinfo-icon, which end on Feb. 20.

"We're in a window when an invasion could begin at any time and, to be clear, that includes during the Olympics," Blinken said.

Russia opened 10 days of massive military drills in Belarus on Thursday and docked six of its ships at a strategic Black Sea port, drawing a sharp rebuke from Ukrainian officials, who characterized Moscowinfo-icon's actions as further escalating tensions in the region.

The Russian maneuvers in Belarus involved thousands of troops and sophisticated weaponsinfo-icon systems, such as S-400 surface-to-air missiles, Pantsir air defense systems and Su-35 fighter jets, with some of the training just 210 kilometers north of the Ukrainian capital, Kyiv.

The Russian Defense Ministry said the six ships arrived at the port of Sevastopol in Crimea, which Moscow annexed from Ukraine in 2014. They had been on a 13,000-kilometer journey from the Baltic Sea to begin what officials described as naval exercises. The Russian landing ships are designed for unloading troops, vehicles and material onto land.

Officials in Moscow and Minsk have said Russian troops will withdraw from Belarus sometime after the drills end February 20. But Western officials remain fearful they could be deployed in a Russian invasion of Ukraine, a onetime Soviet republic, along with 100,000 troops Moscow has massed along Ukraine's eastern flank.

Ukrainian officials, who launched their own drills on Thursday, assailed the impending naval drills, characterizing them as "destructive activity to destabilize the security situation." Kyiv accused Russia of violating international law by restricting wide swaths of open waters to conduct missileinfo-icon and artillery fire training.

"These actions pose another threat to Ukraine's sovereignty in its territorial sea area and in its sovereign rights in the exclusive maritime economic zone," Ukrainian Defense Minister Oleksii Reznikov said in a statement. "By blocking the recommended sea lanes, the Russian Federation has made it literally impossible to navigate in these areas and allow ships to enter Ukrainian seaports, especially in the Sea of Azov."

Kremlin spokesperson Dmitry Peskov, in a call with reporters, denied that the drills would affect seagoing commercial operations.

Peskov said Russia was staging the joint exercises with Belarus, its largest ever, to combat "unprecedented security threats ... the nature and, perhaps, concentration of which are, unfortunately, much larger and much more dangerous than before."

Russian officials have denied they plan to invade Ukraine, but diplomatic talks with Western officials have led to a standoff. Russia has demanded that the United Statesinfo-icon and its allies reject Ukraine's bid for membership in NATO.

The West has rejected that as a non-starter but has said it is willing to negotiate with Moscow over missile deployment and troop exercises in Eastern European countries closest to Russia.

A day of talks between Ukraine and Russia ended Thursday without progress. These talks are an attempt to end the eight-year conflict between Ukraine forces and pro-Russian separatists in the Donetsk and Luhansk regions.

The failure to reconcile each side's interpretations of the Minsk accords, a 2015 agreement that attempted to end fighting in eastern Ukraine, was seen as a setback to defusing the wider crisis.

"I hope that we will meet again very soon and continue these negotiations. Everyone is determined to achieve a result," Ukraine envoy Andriy Yermak said, adding that both sides agreed to keep talking.

Britaininfo-icon on Thursday urged Russia to take a "diplomatic route that avoids conflict and bloodshed" while warning against any Russian moves that undermine Ukraine's sovereignty.

"Fundamentally, a war in Ukraine would be disastrous for the Russian and Ukrainian people and for European security. And together, NATO has made it clear that any incursion into Ukraine would have massive consequences and carry severe costs," British Foreign Secretary Liz Truss said as she met with Russian Foreign Minister Sergey Lavrov.

Western governments have been calling on Russia to take steps to de-escalate the crisis and vowed to impose swift and severe economic sanctions if Russia invades Ukraine.

Lavrov said Thursday that only "mutually respectful dialogue" can lead to normalized relations. "Ideological approaches, ultimatums, threats -- this is the road to nowhere," Lavrov said.

British Prime Minister Boris Johnson traveled Thursday to Brussels to discuss the crisis with NATO Secretary-General Jens Stoltenberg before heading for more meetings with leaders in Poland.

Johnson called the situation the "biggest security crisis that Europe has faced for decades" as he urged solidarity with NATO allies. He told reporters he does not think that Russian President Vladimir Putin has yet decided whether to invade Ukraine, but added, "Our intelligence remains grim."

Stoltenberg told reporters he sent a letter to Lavrov inviting Russia for more rounds of meetings to "find a diplomatic way forward."

"We are prepared to listen to Russia's concerns and ready to discuss ways to uphold and strengthen the fundamental principles of European security that we all have signed up to," Stoltenberg said.

He added, "Renewed Russian aggression will lead to more NATO presence, not less."