In a disturbing escalation of tensions, Vladimir Putin ordered his military command to put nuclear deterrence forces on high alert, in response to what he called “aggressive statements” by Nato countries. In giving the order, the Russian president complained of “illegitimate sanctions” by western countries, which target him personally. It was not immediately clear what his order of “special mode of combat duty” entailed. Putin has warned foreign countries not to interfere in his invasion of Ukraine, saying it could lead to “consequences they have never seen”.

Ukraine claimed its forces had repelled a Russian attempt to seize Kharkiv, the country’s second largest city, following fierce fighting and street battles with advancing Russian troops. Governor Oleg Sinegubov said Russian soldiers were surrendering in groups of five to 10 and throwing their equipment in the middle of the road.

In a decisive break with the past, German chancellor Olaf Scholz told a special session of the Bundestag the government would create a €100bn fund for the armed forces, meaning Berlin will meet a long-standing Nato pledge to spend at least 2% of GDP on defence. Meanwhile, in Berlin, more than 100,000 people protested against Russia’s invasion of Ukraine.

Speaking to the faithful in St Peter’s Square on Sunday, the Pope said those who make war should not be deluded into thinking that God is on their side.

Western allies have agreed to block Russia’s access to the Swift international banking payment system. The US, Canada and key European countries, including Germany, have agreed to remove “selected Russian banks” from the Swift payment system, the countries announced on Saturday.

There were reports that a gas pipeline was on fire in Kharkiv after a Russian attack, while an oil terminal in Vasylkiv, south-west of the capital, Kyiv, has also been targeted. The government has warned that smoke from the explosion in Kharkiv could cause an “environmental catastrophe” and advised people to cover their windows.

Ukraine’s deputy defence minister, Hanna Malyar, has claimed Russian forces have lost about 4,300 servicemen, a figure it was not possible to independently verify.

The 13 Ukrainian soldiers who were reportedly killed while defending an island in the Black Sea from an air and sea bombardment – reportedly telling a Russian navy warship to “go fuck yourself” when asked to surrender – may still be alive, according to Ukrainian officials.

The UN refugee agency has said more than 368,000 people have fled Ukraine into neighbouring countries. Up to 4 million people could become refugees if the situation continues to worsen, the agency has said.

The United Nations security council is due to vote on Sunday to call for a rare emergency special session of the 193-member general assembly on the invasion, diplomats said. The vote needs nine votes in favour and cannot be vetoed. Diploamts say it is likely to pass, diplomats said. Only 10 such emergency special sessions have been convened since 1950.

Turkey referred to the Russian invasion as a “war” on Sunday, marking a change in its rhetoric that could pave the way for Ankara to agree to Kyiv’s request that Russian ships be blocked from passing through the Dardanelles and Bosphorus straits connecting the Mediterranean and Black sea.

Britain is preparing a “hit list” of Russian oligarchs to be targeted by sanctions in the coming months, the foreign secretary, Liz Truss, has said.