The United States said Wednesday it had spotted movement of Russian troops along the Ukrainian border but said it was too soon to tell if a withdrawal promised by President Vladimir Putin was under way.

"I can report to you that we have seen some indications of activity on the border but it's too early to conclude that that activity indicates a withdrawal from the border," White House spokesman Jay Carney said.

U.S. Vice President Joe Biden on Wednesday threatened Russia with tougher sanctions if it undermines a crucial presidential election in Ukraine this weekend.

"If Russia undermines these elections on Sunday, we must remain resolute in imposing greater costs," Biden told reporters in Bucharest after meeting with Romanian president Traian Basescu.

Ukraine plans to deploy more than 55,000 police and 20,000 volunteers to ensure that Sunday's presidential ballot goes off smoothly despite the bloody separatist uprising gripping the industrialized east.

The interior ministry's public order director Andriy Chaliy conceded that the "threat of Russia's aggression and the actions of separatists in the east" posed a threat to Sunday's snap vote.

The U.S. Navy is sending a guided missile cruiser to the Black Sea, the Pentagon said 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.

Russia and Western powers are on the brink of a new Cold War over the crisis in Ukraine, Russian Prime Minister Dmitry Medvedev said in an interview broadcast Tuesday.

"Basically we are slowly but surely approaching a second Cold War that nobody needs," Medvedev said in an interview with Bloomberg Television that was also published on the Russian government's website.

Ukraine said Tuesday that Russian troops had moved away from the border, just five days before the country's make-or-break presidential poll, but stopped short of confirming a full withdrawal as demanded by the West.

Moscow had announced it was pulling back its forces in a move that has the potential to deflate a bloody Kremlin-backed insurgency threatening to tear the ex-Soviet nation apart.

Russian troops ordered by President Vladimir Putin to return to their bases from near the Ukrainian border were preparing their departure on Tuesday, the defense ministry said.

"The commanders in the exercise areas are now clarifying the routes and movement plans" for the troops to leave, Interfax quoted a ministry source as saying.

A top U.N. rights official warned on Monday of a risk of a major exodus from rebel-held areas of east Ukraine because of the near collapse of basic services there.

Ivan Simonovic also cautioned in an interview with Agence France Presse against expecting that Sunday's election would provide a "miracle" that would resolve a crisis that threatens Ukraine's very existence, saying the window of opportunity was closing fast.

Several journalists complained Monday of arrests by Russian security services in Crimea, while Moscow accused Kiev of holding two reporters captive in eastern Ukraine.

Pro-Russian militia and police in Crimea detained several reporters Saturday and Sunday, with one reporter, Osman Pashayev, complaining that he was robbed and kicked while in detention.

Held up by many as the best hope for Ukraine, Sunday's presidential vote will make little difference to the one man the West blames most for the country's crisis -- Vladimir Putin.

Bent on countering Western ambitions in Ukraine, Russia's leader will never accept a vote that cements the authority of Kiev's new government, analysts say.