Russia today pledged it would not invade mainland Ukraine following its seizure of Crimea and said it favoured the ex-Soviet state becoming a federation as a way of defusing the crisis.

Tensions have run high after Russian President Vladimir Putin ripped up the post-Soviet order with Moscow's lightning takeover of Crimea from Ukraine, with the United States accusing Russia of massing tens of thousands of troops on Ukraine's eastern border.

But telephone talks between Putin and US counterpart Barack Obama late yesterday were the latest sign of a slight lessening in tensions between Moscow and the West and a search for a mutual solution in what remains the worst East-West crisis since the Cold War.

Foreign Minister Sergei Lavrov hinted at what were Moscow's main demands in the negotiations -- that Ukraine should be made into a federation and commit to not joining NATO, while order should be restored to the Ukrainian capital Kiev where protesters have thronged the city centre for half a year.

Ukraine is now entering a crucial phase in its development after the fall of pro-Kremlin president Viktor Yanukovych in February, as the clock ticks down to May 25 presidential elections which are expected to cement Kiev's pro-West course.

With boxing champion turned politician Vitali Klitschko bowing out of the race, the overwhelming favourite to win those elections is pro-West confectionary tycoon Petro Poroshenko, a former economy and foreign minister.

Lavrov said in a major Russian television interview broadcast today that Moscow has absolutely no intention of ordering its armed forces to cross over the Ukrainian border and acknowledged the divisions between Moscow and the West on the crisis are narrowing.

"We have absolutely no intention and no interests in crossing the Ukrainian border," he said.

"We (Russia and the West) are getting closer in our positions," he added, saying recent contacts had shown the outlines of a "possible joint initiative which could be presented to our Ukrainian colleagues," he added.