US President Donald Trump said Russia just wanted to "keep killing people" and hinted at sanctions after Moscow launched its largest ever drone and missile attack on Ukraine.
Mr Trump said he was "very unhappy" about his telephone call with Russian leader Vladimir Putin, saying: "He wants to go all the way, just keep killing people, it's no good".
The US president said he and Mr Putin talked about sanctions "a lot".
"He understands that it may be coming," he added.
Largest ever drone attack overnight
Hours-long Russian bombardments sent Ukrainians scurrying for shelters across the country, which came after the call between Mr Trump and Mr Putin, which ended without a breakthrough.
Drones were heard buzzing over the capital and explosions rang out throughout the night as Ukrainian air defence systems fended off the attack.
Tymur, a Kyiv resident who said he had experienced previous Russian attacks said the assault in the early hours yesterday felt different from others.
"Nothing like this attack had ever happened before. There have never been so many explosions," he said.
UN condemns drone attacks on Ukraine
UN Secretary General Antonio Guterres condemned Russia's biggest drone and missile attack yet in the three-year war in Ukraine.
"The Secretary-General strongly condemns the latest series of large-scale drone and missile attacks by the Russian Federation," Mr Guterres said in a statement.
He also called for a full, immediate and unconditional ceasefire.
Zelensky labels Trump call 'most productive yet'
Mr Zelensky said his conversation with Mr Trump yesterday was the best and "most productive" he has had to date.
"Regarding the conversation with the president of the United States, which took place a day earlier, it was probably the best conversation we have had during this whole time, the most productive.
"We discussed air defence issues and I'm grateful for the willingness to help. The Patriot system is precisely the key to protection against ballistic threats," he said.
Mr Zelensky said the two leaders had discussed "several other important matters" that officials from the two sides would be considering in forthcoming meetings.
Increased military aid for Ukraine
Mr Zelensky and Mr Trump said they both agreed to work on bolstering Kyiv's defences.
"We spoke about opportunities in air defence and agreed that we will work together to strengthen protection of our skies," Mr Zelensky said on social media yesterday after the call.
Mr Trump told reporters aboard Air Force One that he had discussed the possibility of sending Patriot air defense missiles to Kyiv.
It comes after the US announced this week it was reducing some of its aid deliveries to Ukraine.
European Commission President Ursula von der Leyen said this was a clear signal that the 27-nation European Union needed to "step up".
Ukraine has also ramped up its retaliatory strikes in Russia, where a woman was killed by a Ukrainian drone attack overnight, the acting governor of the Rostov region said.
Talks, spearheaded by the United States to secure a ceasefire have stalled.
Delegations from the two sides last met more than a month ago, when they agreed to exchange 1,000 prisoners each.
Russia announced a fresh swap of prisoners of war with Ukraine yesterday as part of that agreement
