Russia and Belarus are due to start 10 days of joint military drills as concerns rise over the build-up of Russian forces on Ukraine's borders.

Nato says the joint drills mark Russia's biggest deployment to ex-Soviet Belarus, since the Cold War.

The US called the drills an "escalatory" action in the tensions over Ukraine.

Russia has repeatedly denied any plans to invade Ukraine despite amassing more than 100,000 troops at the border.

But some Western countries including the US have warned that a Russian attack could come at any time.

Diplomatic talks aimed at resolving the conflict are expected across Europe on Thursday.

In 2014 Russia annexed Ukraine's southern Crimea peninsula. Since then there has been a long-running conflict in eastern Ukraine, where Russian-backed separatists control swathes of territory and at least 14,000 people have been killed.

Some 30,000 Russian troops are expected to take part in the drills with Belarus.

Belarus leader Alexander Lukashenko is a firm ally of Russian President Vladimir Putin. The Kremlin backed Mr Lukashenko when huge protests erupted in Belarus in 2020, while most Western countries imposed sanctions and refused to recognise election results widely believed to have been rigged in the long-time leader's favour.

A Kremlin spokesman described the joint drills as serious, saying Russia and Belarus were being "confronted with unprecedented threats".

Russia's EU ambassador Vladimir Chizhov, however, told the BBC that his country still believed diplomacy could help de-escalate the crisis over Ukraine. He said Russian troops currently stationed in Belarus would return to their permanent bases after the exercises.

White House spokesperson Jen Psaki said: "As we look at the preparation for these military exercises, again, we see this as certainly more an escalatory and not a de-escalatory action."