Last month, Zelensky confirmed that Ukraine had used US-supplied long-range missiles for the first time to strike Russian targets in the country's east.
For months, Ukraine has been battling to push back Russian troops who have been slowly advancing in the eastern Donetsk region towards the key city of Pokrovsk - a major supply hub for Ukrainian forces.
Moscow has also massively increased its number of drone strikes on Ukraine. More than 2,000 were launched in October, according to Ukraine’s general staff - a record number in the war.
Overnight on Saturday, Russia launched what is thought to be its biggest co-ordinated assault in months, killing at least 10 people. Around 120 missiles and 90 drones were launched, according to Zelensky.
Attacks continued on Sunday evening, with officials in Sumy region - near the Russian border - reporting another eight killed, including two children, after a missile hit a residential building.
Russian officials in the border region of Bryansk reported a Ukrainian drone attack on Sunday night but said its defences had shot down 26 drones.
Ukraine had for months argued that its allies had not provided the country with enough support to allow it to effectively defend itself.
Joe Biden, who will leave the White House in January, has been seeking to expedite further aid to Ukraine.
There are concerns that his successor, Donald Trump, will slow or halt any further support. He has described the military support as a drain on US resources and has signalled he will end the war, without explaining how this would happen.
The US has been the greatest supplier of arms to Ukraine. Between the start of the war and the end of June 2024, it delivered or committed to send weapons and equipment worth $55.5bn (£41.5bn), according to the Kiel Institute for the World Economy, a German research organisation.
