The UK government is working on "stepping up" sanctions against Russia as reports emerged of alleged civilian killings on the streets of some Ukrainian regions.

Truss has flown down to Poland to meet with her Ukrainian counterpart, Minister of Foreign Affairs Dmytro Kuleba, and Polish foreign minister Zbigniew Rau. It comes ahead of talks involving the North Atlantic Treaty Organisation (NATO) and G7 countries later this week.

UK Prime Minister Boris Johnson said, "Russia’s despicable attacks against innocent civilians in Irpin and Bucha are yet more evidence that Putin and his army are committing war crimes in Ukraine.

"No denial or disinformation from the Kremlin can hide what we all know to be the truth – Putin is desperate, his invasion is failing, and Ukraine’s resolve has never been stronger. I will do everything in my power to starve Putin’s war machine. We are stepping up our sanctions and military support, as well as bolstering our humanitarian support package to help those in need on the ground."

Truss also called for "even tougher action" from allies against Russia to strengthen Ukraine’s negotiating position in peace talks.?

She said, "Putin is yet to show he is serious about diplomacy. A tough approach from the UK and our allies is vital to strengthen Ukraine’s hand in negotiation.

"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. We will continue to support those who are suffering as a result of Putin’s illegal invasion of Ukraine, including the victims of sexual violence and those in need of humanitarian support."

While in Poland, Truss will visit aid organisations supporting Ukrainian refugees. Truss, who returned from a visit to India last week, will then travel to Brussels for NATO talks later in the week as part of a flurry of international visits to build a global coalition against the Russian regime's actions in Ukraine.

Meanwhile, the NATO flag is flying above Downing Street in London as the military alliance marks 73 years since its foundation on Monday.?

The UK government announced that one of the prime minister's top foreign policy advisors, David Quarrey, will be Britain's next Permanent Representative to NATO.?

Quarrey, who was previously the Deputy National Security Advisor, will take forward the work on the response to the unfolding crisis in Ukraine and the future of the Alliance, Downing Street said.

"As members of the greatest security alliance in the history of the world, we have a responsibility to give them [Ukrainians] everything they need to face down this barbaric attempt to subjugate the Ukrainian people," Johnson said in his message marking NATO’s 73rd anniversary.

He added, "As the largest European contributor to the Alliance and as a staunch friend to Ukraine, the UK will continue to do just that."

As part of the NATO response to what the UK government has described as "Putin’s hostility", Britain said it has doubled its troop presence in Estonia, deployed HMS Trent and HMS Diamond ships to the Eastern Mediterranean and sent Royal Air Force (RAF) Typhoons to patrol the skies of Romania and Poland.

With AP inputs