Russian troops encircled the Ukrainian port city of Mariupol on Tuesday while cutting off Ukrainian access to the Azov Sea, according to a TASS news agency report quoting Russia's Defense Ministry.

The forces of Russian-backed separatists have reached the borders of Ukraine's Donetsk province and joined Russian troops, another agency, RIA, quoted the ministry as saying.

Separatist forces in Donetsk earlier stated they have established two corridors for the evacuation of civilians from Mariupol, an indication that a large attack on the key Azov Sea port could be imminent.

Mariupol came under intense shelling on Tuesday, leaving the key southeastern Ukrainian city without power as electricity has been cut off.

Defense ministry spokesperson Igor Konashenkov said that on Tuesday, "the Russian army carried out strikes with long-range high-precision weapons from the sea," without specifying the targeted area.

He said that "two airfields and three air defence radars were hit" in Ukraine, adding that no civilian infrastructure was targeted.

Eduard Basurin, a spokesperson for the separatists' military, said on Tuesday that civilian safety of movement is guaranteed until Wednesday in the corridors.

Mariupol, an industrial center, is seen as a key target for Russian forces for its economic value and its location, which would help Russia establish a land corridor between Crimea, the Donbass region and the Russian mainland.

Russian troops had earlier captured two other important towns near Azov, Melitopol and Berdyansk, and closed in on regional capital Zaporizhzhia through the south and southwest.

Black Sea ports Kherson and Mykolaiv are currently under Russian control with clashes underway in the latter.

These developments left the major port city Odessa, its suburb Chernomorsk and the small port town Ochakiv as Kyiv's only access to seas. All three ports face Russian-occupied Crimea to their east and south and are threatened by the Russian troop presence from the east in Mykolaiv and west from Transnistria, a Russia-backed breakaway region in Moldova.

Russian troops annexed the Crimean peninsula from Ukraine in 2014 and have used the region to launch attacks since Thursday when Moscow began its invasion.

Pro-Kremlin separatist fighters in east Ukraine have been pushing west as part of the offensive.