LONDON: Russian President Vladimir Putin could drag out the Ukraine crisis for months in an attempt to challenge Western unity, British Foreign Secretary Liz Truss said.

"There is currently no evidence the Russians are withdrawing from border regions near Ukraine," Truss wrote in The Daily Telegraph newspaper. "The Russian military build-up shows no signs of slowing."

"We must have no illusions that Russia could drag this out much longer in a brazen ploy to spend weeks more – if not months – subverting Ukraine and challenging Western unity," Truss said.

Russia is continuing to prepare for a possible invasion and there have been bridges put across the Pripyat River near the Belarus-Ukraine border, James Heappey, Britain minister for the armed forces, told BBC Radio.

Russia says the West is gripped by hysteria and Russophobia, and Moscow has ridiculed claims by U.S. security chiefs that an invasion of Ukraine is imminent.

The Russian defence ministry on Tuesday published footage to demonstrate it was returning some troops to base after exercises.