The United States is sending another warship to the Black Sea to reassure its allies over Russia's actions in Ukraine, where the country holds a crucial presidential election at the weekend.

Washington also said Tuesday there was no sign of a Russian withdrawal of troops from the edge of Ukraine, despite Moscow's announcement of a pullout and suggestions from Kiev that there was no longer any visible military activity in the border zone.

"They are still in the tens of thousands," Pentagon spokesman Rear Admiral John Kirby told reporters, announcing the dispatch of a guided missile cruiser to the region. "We have not seen any withdrawal activity."

The US and its allies have warned the Kremlin of further sanctions over the crisis in Ukraine, where a bloody pro-Moscow insurgency in the east is threatening to tear the ex-Soviet nation apart.

US Vice President Joe Biden, on a visit to Romania, said Wednesday the West "must remain resolute in imposing greater costs to Russia" if it undermines Ukraine's elections.

Ahead of Sunday's vote, Kiev's Western-backed leaders are hosting a new round of national unity talks under a peace plan sponsored by the Organisation for Security and Cooperation in Europe.

But an earlier two rounds failed to make any progress, with the government pointedly refusing to invite the separatists who have seized over a dozen towns in the industrial eastern belt.

Tensions between Moscow and the West have spiralled to Cold-War highs over the crisis in Ukraine, particularly Russia's annexation of Crimea in March and allegations it is driving the insurgency.

- Ballistic missile test -

The United States and NATO have sent troops to Poland and the three tiny Baltic nations to calm jitters about Russian troops possibly not only overrunning Ukraine but also pushing further into Europe in a bid to reclaim ex-Soviet satellite states.

In its own show of force Tuesday, Russia said it carried out a successful test-launch of an intercontinental ballistic missile from a site near the Caspian Sea.

Moscow had announced Tuesday that its soldiers were preparing to head back to their bases from the Ukrainian border.

And Ukraine's own border service said none of the estimated 40,000 Russian troops were now stationed within 10 kilometres (six miles) of the country.

But Foreign Minister Andriy Deshchytsia, on a visit to Berlin, later said he could not confirm a withdrawal.

"I hope that the declarations by Russian politicians that the troops are to be withdrawn from Ukraine's borders do not just remain declarations," he said.

NATO chief Anders Fogh Rasmussen said this week that a real withdrawal -- following several earlier promises by President Vladimir Putin -- would be an "important contribution to de-escalating the crisis".

But the war of words between Washington and Moscow showed no signs of a let-up.

Russian Prime Minister Dmitry Medvedev said in an interview that Moscow and the West were still "slowly but surely" approaching a second Cold War.

And Biden blasted Russia's seizure of the Crimean peninsula, saying: "Europe's borders should never again be changed at the point of a gun".

Both Kiev and its Western allies see Sunday's vote -- backed only grudgingly by Moscow -- as a chance to unite the culturally splintered nation and win more legitimacy in the Kremlin's eyes.

The front-runner, chocolate baron Petro Poroshenko, has pledged to find a negotiated solution to the crisis and rebuild ties with Ukraine's former masters in Moscow.

The United Nations estimates that around 130 people have died since the insurgency began last month in the Donetsk and Lugansk regions, where rebels have declared independence.

In nearly six weeks of fighting, Ukraine's military has so far failed to dislodge the rebels and suffered several humiliating setbacks since it launched its "anti-terrorist" offensive.

Kiev authorities have admitted they will have a hard time ensuring that polling proceeds smoothly in eastern districts controlled by the armed insurgents.

But in a move that some say could turn the tide against the rebels, Ukraine's richest man Rinat Akhmetov has condemned the rebellion in his industrial powerbase.

"People are tired of living in fear and terror," said Ahkmetov, a coal and steel magnate who once backed the deposed pro-Kremlin regime but is now seeking to build closer relations with the new team in Kiev.

The Ukrainian government hailed Akhmetov's intervention, with Interior Minister Arsen Avakov saying it "will help (Ukrainians) settle our differences and let our rifles gather dust".

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