The Prime Minister reaffirmed the UK's support for Ukraine, in a call with Ukrainian President Volodymyr Zelensky.
The two leaders spoke after a Ukrainian rocket attack killed dozens of Russian soldiers in a town in the Donetsk region.
Russia had unleashed a barrage of attacks on Ukrainian sites over the New Year period.
Rishi Sunak told the Ukrainian President that the country could "count on the UK to continue to support it for the long term".
A Downing Street spokesperson said: "The leaders discussed the abhorrent drone attacks on Ukraine in recent days, and the Prime Minister said the thoughts of the UK were with the Ukrainian people as they continued to live under such bombardment.
"The Prime Minister said Ukraine could count on the UK to continue to support it for the long term, as demonstrated by the recent delivery of more than 1,000 anti-air missiles.
"Work was also under way to provide further equipment in the coming weeks and months to secure Ukraine's victory on the battlefield, the Prime Minister added."
The two leaders discussed the recent Joint Expeditionary Force summit in Latvia last month.
"The leaders agreed to stay in close touch in the coming weeks," the spokesperson said.
In a tweet, Mr Zelensky spoke of hopes to "bring victory closer" in 2023, with the support of the UK.
He tweeted that together with Rishi Sunak "we discussed further defence co-operation".
"We agreed to intensify our efforts to bring victory closer this year already. We already have concrete decisions for this," the Ukrainian President said.
Rishi Sunak met with President Zelensky during his first visit to Ukraine last November, in a signal of continuing UK support for the Ukrainian President.
The Ukrainian leader now appears to be starting the new year with a round of phone calls to European leaders and allies, including European Commission President Ursula von der Leyen.