Truss's visit comes as the world reacts to images from Bucha showing the bodies of civilians lying on the ground and makeshift graves after the town in northern Ukraine was retaken from Russian troops.

The Kremlin categorically denied any accusations related to the murder of civilians in the town. Moscow has denied targeting civilians and has said similar reports of killings were "staged" to sully Russia's name.

"Reports of shallow graves outside Kyiv are truly horrifying," Truss said on Twitter. "We will not rest until Russia has paid the price for these appalling crimes."

As part of her visit to Poland, Truss is due to meet Ukrainian foreign minister Dmytro Kuleba in Warsaw on Monday and her Polish counterpart Zbigniew Rau on Tuesday.

"Britain has helped lead the way with sanctions to cripple the Putin war machine. We will do more to ramp up the pressure on Russia and we will keep pushing others to do more," she said in a separate statement announcing her trip.

"A tough approach from the UK and our allies is vital to strengthen Ukraine's hand in negotiations," added Truss, who will head to Brussels later this week for talks with the G7 and NATO.

Russia, which invaded Ukraine on Feb. 24, says it is conducting a "special military operation" with the aim of demilitarising its neighbour. Ukraine and its Western allies call this a pretext for an unprovoked invasion.