KYIV: Top US diplomat Antony Blinken said on an unexpected visit to Kyiv Wednesday that Washington would "maximise" efforts to support Ukraine as it pressed ahead with its counteroffensive against Russia.

His comments came hours after officials said a Russian artillery strike on a market in eastern Ukraine killed over a dozen people, in one of the deadliest strikes in weeks.

"President Biden asked me to come to reaffirm strongly our support, to ensure that we are maximising the efforts that we're making," Blinken told Ukrainian President Volodymyr Zelensky.

"We see the important progress that's being made now in the counteroffensive and that's very, very encouraging," he said.

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Ukraine launched its counteroffensive against Russia in June after building up Western-supplied weapons, but has repeatedly hit back at criticism for its apparent slow pace and limited gains.

During the meeting with Blinken, Zelensky thanked the US for its support, warning that Ukraine expected difficult months ahead as winter approached.

"We're happy that we're not alone through this winter, we will do it together with our partners," he told Blinken.

Blinken's visit was overshadowed by the attack on Kostiantynivka -- an eastern town of nearly 70,000 people about 20 kilometres (12 miles) from the front line.

Zelensky, who denounced Wednesday's attack as "heinous", has repeatedly warned Russia is targeting civilian infrastructure as the counteroffensive rages on.