Ukraine on Wednesday dismissed Russian proposals to turn it into a neutral state, as the United States readied a huge package of military aid ahead of a historic address to Congress by Kyiv's defiant leader.
On the ground, Russian rocket fire hit a train station in southern Ukraine used by thousands of fleeing refugees and fresh blasts hit the capital, which was under a total curfew.
Biden and other NATO leaders have been stepping up military support for Ukraine including anti-tank weapons that have helped to stall Russian forces north of Kyiv.
Regional officials reported no casualties as Russian strikes blew out windows at the railway station in Zaporizhzhia, where people have been arriving from the besieged city of Mariupol.
Some 20,000 residents have been allowed to leave Mariupol. But exhausted, shivering evacuees speak of harrowing escape journeys and rotting corpses littering the streets.
"This is the first time I have been able to breathe in weeks," said Mykola, asking not to give his full name, after driving his wife and two young children through a minefield to avoid Russian checkpoints.
The conflict has already sent more than three million Ukrainians fleeing across the border, and a peaceful resolution still seems beyond reach.
Kyiv braced for an offensive from encircling Russian forces, although their progress has been slow due in part to resistance from Ukrainian troops using high-tech Western weaponry.
The city was subjected to a fresh round of bombardment overnight close to its center, including on a 12-story residential building, with columns of smoke rising high into the morning sky.
Ukraine's State Emergency Service said at least two people were injured and 37 evacuated from the apartment block.
The attacks came as Ukraine and Russia continued stop-start peace talks, after Ukrainian President Volodymyr Zelensky conceded that NATO membership was not on the table, in an apparent concession to Moscow.
Kremlin spokesman Dmitry Peskov said a "compromise" outcome would center on Ukraine becoming a neutral state comparable to Sweden and Austria.
But Zelensky's office gave the idea short shrift.
"Ukraine is now in a direct state of war with Russia. Consequently, the model can only be 'Ukrainian' and only on legally verified security guarantees," Kyiv's negotiator Mikhailo Podolyak said.
In an earlier overnight video address, Zelensky said "all wars end with an agreement" as he pointed to the "difficult" but "important" ongoing round of talks.
But in a landmark virtual speech to the US Congress, the president was also expected to intensify pleas for NATO allies to intervene to stop Russia's nearly three-week-old invasion.
The alliance will hold an emergency summit next week in Brussels with US President Joe Biden attending -- but it has so far resisted Zelensky's pleas for direct involvement for fear of starting World War III.
Indirectly, however, Biden and other NATO leaders have been stepping up military support for Ukraine including anti-tank weapons that have helped to stall Russian forces north of Kyiv.
Coinciding with Zelensky's speech, a White House official said Biden would unveil another $800 million of military aid, expected to include more anti-tank and anti-aircraft missiles.
After a morale-boosting train journey to Kyiv by the leaders of Poland, the Czech Republic and Slovenia, International Committee of the Red Cross president Peter Maurer came for a five-day visit.
He noted the "enormous suffering by the civilian population" in Ukraine and stressed it was "utterly important" for the ICRC's neutral humanitarian work to be recognized on the ground.
Punishing Western sanctions against President Vladimir Putin's regime have stepped up this week, pushing Russia to the brink of default on its foreign debts.
Moscow was due Wednesday to pay $117 million on two dollar-denominated bonds -- the first interest payment that has fallen due since it was largely shut out of the Western financial system.
Putin has turned to Chechnya and Syria to replenish his invasion force -- and to China for economic support.
'Plan C' for Russia?
But facing stern US threats against helping Russia to bypass the sanctions, foreign ministry spokesman Zhao Lijian said Beijing was acting "objectively and impartially".
"We urge the US not to damage China's legitimate rights and interests in any form. If the US continues down this path, China will certainly take strong countermeasures," he added.
Western military experts believe Russia is increasingly turning to aerial bombardments after an initial ground invasion stalled -- and as possible leverage in the negotiations.
"They have found that their ground operations are not succeeding very well and where they are making gains they are at massive costs that are not sustainable," Mick Ryan, a retired Australian major general, told AFP.
"They have had to change to 'Plan C' -- which is to bombard cities and terrorize civilians in the hope that the Ukrainians will reach some kind of political accommodation," he said.
"What the Russians are doing is using our own humanity against us and Zelensky's humanity against him."
The conflict has also raised acute fears of Europe-wide fallout if Ukraine's nuclear power stations suffer serious damage.
But Belarus said it was now supplying electricity to Ukraine's Chernobyl nuclear power station, the site of the world's worst nuclear disaster in 1986.