The foreign secretary said "crippling" sanctions are pushing the Russian economy back "into the Soviet era".

More than $350bn (£266bn) of Russia's $604bn foreign currency reserves are unavailable to the regime, she added.

Her call for more to be done comes amid condemnation after images of bodies in the streets of Bucha, near Kyiv, emerged after Russian troops withdrew.

Ukrainian President Volodymyr Zelensky has warned he believes the worst atrocities committed by Russian forces are yet to be discovered, but Russia has denied killing civilians - claiming, without evidence, that Ukraine has staged such scenes.

Prime Minister Boris Johnson has released a video on social media, addressed to the Russian public, in which he accuses Russian troops of committing atrocities, including the massacre of civilians and the raping of women.

Speaking in Russian at the close of the video, he says: "Your president stands accused of committing war crimes. But I cannot believe he's acting in your name."

Ms Truss, who spoke after talks with her Polish counterpart Zbigniew Rau, urged G7 countries to go further in their sanctions ahead of G7 and Nato meetings this week.

Measures she is calling for include banning Russian ships from their ports, cracking down on Russian banks, going after industries "filling Putin's war chest" such as the gold trade, and agreeing a timetable to eliminate imports of Russian oil and gas.

Ms Truss said the only way to end the war is for Russian President Mr Putin to lose in Ukraine and will stress the urgency of stepping up sanctions, as well as giving weapons to Ukraine to defend itself.

"Although Russian troops have been defeated in their initial assault on Kyiv, there has been no change in their intent and ambition," she said.

"We are seeing Putin's forces set their sights on the east and south of Ukraine, with the same reckless disregard for civilian lives and their nationhood.

"So far our sanctions have had a crippling impact on those who feed and fund Putin's war machine."

Both the EU and the US are planning more sanctions on Russia this week, with EU ambassadors meeting on Wednesday to decide what steps to take.

European Commission President Ursula von der Leyen has outlined a fifth wave of sanctions, including an import ban on Russian coal, which she said was worth 4bn euro (£3.34bn) per year.

Earlier, Ms Truss agreed with her equivalent from Japan, a fellow G7 member, that the international community must increase pressure on Russia with further co-ordinated sanctions.

The UK has also announced a £10m fund to support organisations in Ukraine, including those helping victims of conflict-related sexual violence.

The UK will also be providing funding and technical assistance for the International Criminal Court's investigation into reports of rape.

"We have all been shocked by the scenes from Bucha," she said. "These are appalling acts of the kind that we thought we left in the 20th Century."

A satellite image of the town from 19 March appears to show bodies lying in the street nearly two weeks before Russians left Bucha.

The picture directly contradicts Russian Foreign Minister Sergei Lavrov's claim that footage of bodies in Bucha was "staged" after the Russians withdrew.

It shows objects that appear to be bodies in the precise locations where Ukrainian forces subsequently found them when they regained control of the town.

During the foreign secretary's visit she also praised Poland for being on the "front line of helping Ukraine" and for always being "clear-eyed" about "Putin's malign intent".

Ukraine's Foreign Minister Dmytro Kuleba has called for countries to stop buying oil, gas and coal from Russia to avert "new Buchas". He described the move as the "mother of all sanctions", saying it would stop the war in a matter of months.