US Defense Secretary Lloyd Austin urged Russian Defense Minister Sergei Shoigu on Friday to move immediately to implement a ceasefire in Ukraine, in their first conversation since before the war began, the Pentagon said.

"Secretary Austin urged an immediate ceasefire in Ukraine and emphasized the importance of maintaining lines of communication," the Pentagon said in a statement.

The Pentagon gave no other details on the content of the conversation, their first since February 18, six days before Russian forces invaded Ukraine.

The call came as the two sides battle along a long front line in eastern and southern Ukraine, with the Pentagon maintaining that Russia is weeks behind goals set in its war plan.

But Moscow has shown no indication of pulling back and is believed by Western intelligence of wanting to take control of a wide swath of southern Ukraine stretching along the Black Sea to Moldova.

The call also came as Finland and Sweden have expressed the desire to join the NATO defense alliance, a direct consequence of Russia's attack on Ukraine, which is not part of NATO.