US Secretary of Defense Lloyd Austin called on Friday in an appeal to his Russian counterpart Sergei Shoigu for an "immediate" ceasefire in Ukraine, but no "acute problems" could be resolved during their exchange, said announced the Pentagon.

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Austin "urged an immediate ceasefire in Ukraine and stressed the importance of maintaining lines of communication," Pentagon spokesman John Kirby said in a statement.

The two leaders exchanged "for the first time since February 18", a few days before the outbreak of the conflict in Ukraine, he added, without giving further details of the conversation.

According to a senior Pentagon official, "the call itself did not resolve any acute issues or lead to direct changes in what the Russians do or say."

In a very short statement, the Russian Defense Ministry claimed that the call took place "at the initiative of the American side", and that the two ministers had "discussed current issues of international security, including of the situation in Ukraine".

This call comes as the advance of Russian forces in the Donbass against Ukrainian troops is several weeks behind the Kremlin's plans, according to Washington.

But Moscow has given no indication of slowing down or withdrawing and, according to Western intelligence, wants to take control of a large part of Ukrainian territory, in the south and east of the country.

Finland and Sweden this week expressed their desire to join NATO, a direct consequence of the Russian invasion of Ukraine, which is not a member of the Atlantic Alliance.