Russia's defence ministry said on Friday that the first phase of its military operation in Ukraine was mostly complete and that it would focus on completely "liberating" eastern Ukraine's Donbass region.

The announcement appeared to indicate that Russia may be switching to more limited goals after running into fierce Ukrainian resistance in the first month of the war.

Russian news agencies quoted the defence ministry as saying that Russian-backed separatists now controlled 93% of Ukraine's Luhansk region and 54% of the Donetsk region - the two areas that jointly make up the Donbass.

The ministry said it did not rule out storming Ukrainian cities that had been blockaded and that Russia would react immediately to any attempt to close the airspace over Ukraine - something Kyiv has asked NATO to do, but NATO has resisted.

Russia sent tens of thousands of troops into Ukraine on February 24 in what it called a "special operation" to weaken its southern neighbour's military capabilities and root out people it called dangerous nationalists.

Ukrainian forces have mounted stiff resistance to the invasion and the West has imposed sweeping sanctions on Russia in an effort to force it to withdraw its forces.

The defence ministry said on Friday that the operation would continue until Russian forces had completed the tasks that had been set, without elaborating.

Russia's military had considered two options for its operation in Ukraine, one confined to the Donbass and the other on the whole territory of Ukraine, the defence ministry continued.