WASHINGTON (AFP) - The US Navy is sending a guided missile cruiser to the Black Sea, the Pentagon said on Tuesday, the latest bid by Washington to to reassure allies worried over Russia's intervention in Ukraine.

"I can confirm the Vella Gulf, a Navy cruiser, will be going in to the Black Sea probably later this week," Pentagon spokesman Rear Admiral John Kirby told reporters.

The Vella Gulf will arrive in the Black Sea after the recent departure of the frigate USS Taylor, which left the area on May 12.

Since the crisis in Ukraine began in March, the United States has deployed troops to Eastern European countries for joint drills and sent ships to the Black Sea for exercises with Nato states in the area.

But the 1936 Montreux Convention that governs the Black Sea bars outside countries from keeping warships in the strategic waters more than 21 days.

The Pentagon, keen to allay the concerns of allies bordering Russia, has said it would keep up regular deployments of troops to Nato members in Eastern Europe through the end of the year.

The higher US military profile also would "include naval presence in and out of the Black Sea," according to Rear Admiral Kirby.

He also said there was no sign of a Russian withdrawal of troops from the Ukrainian border, despite an announcement of a pullout from Moscow.

"They are still in the tens of thousands," Rear Admiral Kirby said. "We have not seen any withdrawal activity," he said.

Ukraine said earlier that Russian troops had moved away from the border, but stopped short of confirming a full withdrawal as demanded by the West.

Ukraine's border service issued a surprise announcement early on Tuesday that none of the estimated 40,000 Russian soldiers were now stationed within 10 kilometres (six miles) of the country.