Russian President Vladimir Putin threatened the existence of Ukrainian statehood as his army's invasion of the neighbour faces stiff resistance Sunday and his economy is increasingly asphyxiated by sanctions.

In the latest efforts to freeze Moscow out of the world economy, US-based card payment giants Visa and Mastercard announced they will suspend operations in Russia, while world leaders vowed to act over the intensifying onslaught.

"The current (Ukrainian) authorities must understand that if they continue to do what they are doing, they are putting in question the future of Ukrainian statehood," Putin said on Saturday.

"And if this happens, they will be fully responsible." Since Russia's invasion 10 days ago, the economic and humanitarian toll of the war has spiralled, sending more than one million people fleeing Ukraine. Officials have reported hundreds of civilians killed.

In a Facebook post on Sunday the Ukraine military said it was engaged in "fierce battles" with Russian forces for the control of borders at the southern city of Mykolaiv and the Chernihiv in the north.

"The main efforts are focused on defending the city of Mariupol," it said, adding an operation by Ukrainian forces was also under way in the eastern part of the Donetsk region.

Kyiv has urged the West to boost military assistance to the besieged country, including warplanes, with President Volodymyr Zelensky pleading for Eastern European neighbours to provide Russian-made planes that his citizens are trained to fly.

Putin meanwhile escalated warnings against NATO, threatening a wider war if a no-fly zone is set up, as his forces resumed their offensive against a key Ukrainian city where security fears stalled a planned evacuation.

While Zelensky criticized NATO for ruling out the no-fly zone, Putin spoke of "colossal and catastrophic consequences not only for Europe but also the whole world" if such a step was taken.

"Any movement in this direction will be considered by us as participation in an armed conflict by that country," Putin said.

Hitting out at stiffening Western sanctions, the Russian leader said: "A lot of what we're coming up against right now is a way of waging war against Russia.

"The sanctions against Russia are akin to a declaration of war. But thank God we're not at that point yet."

Putin also dismissed rumours that the Kremlin was planning to declare martial law in Russia.

Cards Cut - Visa and Mastercard both announced they will suspend operations in Russia, the latest major American firms to join the business freeze-out of Moscow.

Mastercard said it made the decision over the "unprecedented nature of the current conflict and the uncertain economic environment". Visa meanwhile said that "effective immediately" it would "work with its clients and partners within Russia to cease all Visa transactions over the coming days".

Visa and Mastercard had already announced that they were complying with US and international sanctions imposed on Russia in the wake of its attack.

But Russia's major banks -- including its largest lender Sberbank and the Russia Central Bank -- downplayed the effects that the cards' suspensions would have on their clients.

The war has already had serious global economic impacts, with the IMF warning that its effects would be "all the more devastating" should the conflict escalate.

Russia's business and other contacts with the West have been steadily cut. Moscow has suspended all flights by flagship carrier Aeroflot, effective Tuesday.

Frenzied Diplomacy - As frantic, top-level diplomatic talks continued, President Zelensky announced on Sunday that he spoke by phone with his US counterpart Joe Biden to discuss financial support and sanctions against Russia.

"The agenda included the issues of security, financial support for Ukraine and the continuation of sanctions against Russia," Zelensky tweeted.

Hours earlier, the Ukrainian leader had addressed US lawmakers by video call, pleading for further funding and an embargo on Russian oil imports. The American legislators promised an additional $10 billion aid package, but the White House has so far ruled out an oil ban, fearing it would ratchet up prices and hurt US consumers already stung by record inflation.

Weapons, ammunition and funds have poured into Ukraine from Western allies as they seek to bolster Kyiv against Moscow's invasion.

Washington last week authorised $350 million of military equipment -- the largest such package in US history.