The humanitarian crisis in Ukraine deepened Monday as Russian forces intensified their shelling and food, water, heat and medicine grew increasingly scarce, in what the country condemned as a medieval-style siege by Moscow to batter it into submission.

The third round of talks between the two sides ended with a top Ukrainian official saying there had been minor, unspecified progress toward establishing safe corridors that would allow civilians to escape the fighting. Russia’s chief negotiator said he expects those corridors to start operating Tuesday.

But that remained to be seen, given the failure of previous attempts to lead civilians to safety amid the biggest ground war in Europe since World War II.

Well into the second week of the invasion, with Russian troops making significant advances in southern Ukraine but stalling in some other regions, a top U.S. official said multiple countries were discussing whether to provide the warplanes that Ukrainian President Volodymyr Zelenskyy has been pleading for.

Russian President Vladimir Putin’s forces continued to pummel cities with rockets, and fierce fighting raged in places.

In one of the most desperate cities, the encircled southern port of Mariupol, an estimated 200,000 people — nearly half the population of 430,000 — were hoping to flee, and Red Cross officials waited to hear when a corridor would be established.

The city is short on water, food and power, and cellphone networks are down. Stores have been looted as residents search for essential goods.