Belarusian leader Alexander Lukashenko said Thursday that Russia had begun moving nuclear weapons to its territory, which borders the European Union, ratcheting up tensions with the West over the Ukraine conflict.

On the diplomatic front, Russia's foreign ministry announced a visit to Moscow by China's special envoy for Ukraine, Li Hui -- Beijing's latest effort to broker an end to the fighting.

Li's visit to the Russian capital, where he is expected to meet foreign minister Sergei Lavrov on Friday, comes after he met with Ukraine's President Volodymyr Zelensky in Kyiv.

"The transfer of nuclear munitions has begun," Lukashenko told reporters during a visit to Moscow to meet with President Vladimir Putin.

In Washington, White House Press Secretary Karine Jean-Pierre called Russia's move "yet another example of making irresponsible and provocative choices".

She added, however, that "we have not seen any reason to adjust our own nuclear posture... nor any indication that Russia is prepared to use nuclear weapons from Belarus".

There was no immediate confirmation by Russia of the nuclear weapon transfer.

Lukashenko has allowed his territory, which borders Ukraine as well as EU and NATO members Poland and Lithuania, to serve as a staging ground for Russia's Ukraine offensive.

In March, Putin said he would station "tactical" nuclear weapons -- in effect short-range atomic missiles -- in Belarus, drawing condemnation from the West.

-- US bombers intercepted -

Adding to the global worries, Russia said Thursday that it had sent fighter jets to stop two US bomber planes from "violating the state border" over the Baltic Sea, the second such incident this week.

The Russian defense ministry said it had scrambled Su-27 and Su-35 fighter jets, saying it identified "the air targets as two US Air Force B-1B strategic bombers".

After the maneuver, the Russian fighters "returned safely to their air base", it said.

In Japan, Tokyo's military also said it deployed fighter jets against Russian planes traveling along parts of its coastline.

Ahead of the visit by China's envoy Li to Moscow, the European Union told him on Thursday that it expected Beijing to help pressure Russia to stop the bloodshed in Ukraine and withdraw its troops from the country.

Senior EU foreign policy official Enrique Mora met Li in Brussels on the latest leg of a European tour by Beijing's special representative for Eurasian affairs.

Russia and China have close relations, with Chinese President Xi Jinping visiting Moscow in March and saying their bilateral ties were "entering a new era".

While China says it is a neutral party in the Ukraine conflict, it has been criticized for refusing to condemn Moscow for its offensive.

Battle for Bakhmut

Top US officer General Mark Milley said Thursday that Russia would not achieve a military victory in Ukraine, while also cautioning that Kyiv was unlikely to force out all of Moscow's troops anytime soon.

His comments underscored forecasts that the war in Ukraine is set to drag on, with neither side positioned to win a clear-cut victory and no negotiations currently taking place.

"This war, militarily, is not going to be won by Russia. It's just not," Milley told journalists after a video meeting of dozens of countries that support Ukraine.

On the ground, the head of Russia's Wagner mercenary group said Thursday that his troops had started handing over their positions in the flashpoint eastern Ukraine city of Bakhmut to the Russian military.

Wagner said it began pulling out its fighters after claiming to have fully captured the destroyed city at the weekend.

Ukraine, which denies that Bakhmut has fallen to Russia, insisted it still controlled a "micro district" of the city and said it was still advancing on its flanks.

The battle for Bakhmut has raged for nearly a year, leveling the city and decimating waves of Wagner recruits who have led Russia's assault on the industrial hub.

"We are withdrawing units from Bakhmut today. We are handing over positions to the military, ammunition and everything," Wagner's boss Yevgeny Prigozhin said in a video.

Earlier this week, he conceded that around 10,000 prisoners that Wagner had recruited to fight in Ukraine had been killed on the battlefield.

Drone barrage

In Kyiv, Zelensky accused Russia of terrorizing Ukrainians by launching another wave of Iranian-made attack drones at targets across his country.

"The enemy continued to terrorize Ukraine by launching 36 Shaheds. None reached their target," Zelensky said on social media.

"I'm grateful to our air defense forces for the 100 percent result."

Kyiv has become increasingly adept at taking down Russian cruise missiles and drones after appealing to Western allies for greater air-defence capabilities.

In Moscow, Russia's FSB security service said it had arrested two Ukrainians who had allegedly planned to target nuclear power plants in the country.

© Agence France-Presse