Ukraine President Volodymyr Zelensky has threatened to pull out of peace talks if Russian forces kill the last defenders who have been holed up for days in Mariupol.

A group of marines, border guards, police and members of the controversial Azov regiment are bunkered down in the besieged city’s Azovstal steelworks.

More than 1000 civilians are also cornered with the Ukrainian troops there, according to Ukrainian authorities.

A video circulating purports to show women and children among those sheltering in the damaged steelworks and running out of food and water.

Mr Zelensky told journalists: “If our men are killed in Mariupol…Ukraine will withdraw from any negotiation process”.

The president also said peace talks would be off if Russia attempted to hold a “pseudo” independence referendum in the occupied area of Kherson.

Mr Zelensky told a news conference US Secretary of State Antony Blinken and Secretary of Defence Lloyd Austin would meet with him in Kyiv to discuss the weapons Ukraine needs.

It comes as at least five civilians — including a three-month-old baby —were reported to have been killed in a missile attack the city of Odesa on the Black Sea.

An adviser to Ukraine’s interior minister earlier said Russian forces fired at least six cruise missiles which hit various targets including residential buildings.

Attacks on last defenders

Russia has been accused of trying to “strangle the final resistance of Mariupol’s defenders” as it resumed its assault on the steelworks.

Ukraine presidential adviser Oleksiy Arestovych said Ukrainian troops in the Azovstal complex were still holding out “despite the very difficult situation” and were attempting counterattacks.

Mr Arestovych said Russian forces were hitting the steelworks with air strikes and trying to storm it.

Another attempt to evacuate a group of 200 citizens trapped in the broader city area were “thwarted” by the Russian military, said an adviser to Mariupol’s mayor.

Ukraine estimates tens of thousands of civilians have died in Mariupol and says 100,000 civilians are still there. The United Nations and Red Cross say the civilian toll is at least in the thousands.

A satellite image of the Azovstal steelworks in Mariupol shows holes in the roof. Photo: AAP

The biggest battle of the conflict has raged for weeks as Russia seeks to capture a city seen as crucial to its attempts to link the eastern Donbas region with Crimea which Moscow seized in 2014.

On Saturday Luhansk governor Serhiy Gaidai said Ukrainian forces were pulling back from some settlements to new defensive lines to preserve their units in the face of an intensifying barrage on all cities in the region.

“It’s unpleasant they’re leaving our settlements, but it is no catastrophe,” Mr Gaidai added.

The governor of Kharkiv, a heavily bombarded city northwest of Donbas, said Russian forces had carried out over 50 artillery or rocket attacks in the area over the past day, killing two people and wounding 19.

Governor Oleh Synegubov said intense fighting continued at the frontline town of Izyum, southeast of the city, and that Ukrainian forces had retaken three villages to its northwest.

Russia said on Saturday it had shot down a Ukrainian fighter jet and destroyed three Ukrainian helicopters at an airfield in Kharkiv.

Russian forces made no major gains in the last 24 hours, British military intelligence said on Saturday.

On Friday, Russian general Rustam Minnekayev said Moscow wanted control not just of the Donbas but also over southern Ukraine, suggesting wider goals for the invasion than Russia has previously acknowledged.

Mr Zelensky said after Minnekayev’s comments that Russia’s invasion was just the beginning and that Moscow has designs on capturing other countries.

“We are the first in line. And who will come next?” he said in a video address late on Friday.

-with AAP