US warns Vladimir Putin as he takes war to the West

The US warned Russia that it would face Nato's "full force" if attacks on Ukraine strayed beyond the border after Vladimir Putin brought his war to within 15 miles of Poland with a missile strike on foreign fighters.

Russia boasted that it had killed "up to 180" foreign fighters and destroyed a cache of weapons donated by the West after using long-range missiles to target a military training centre near the city of Lviv hosting volunteer fighters from abroad.

Jake Sullivan, the White House national security adviser, suggested that Nato would engage in direct conflict with Russia if Moscow accidentally hit Nato territory in Poland.

"If there is a military attack on Nato territory, it would ... bring the full force of the Nato alliance to bear in responding to it," he said.

Volodymyr Zelensky, the Ukrainian president, later reiterated calls for a no-fly zone over his country, warning: "If you don't close our sky, it is only a matter of time before Russian rockets fall on your territory, on Nato territory."

It came as Michael Gove, the Cabinet minister, warned that Putin was "pushing the boundaries" in warfare as he accused the Russian president of having "no moral limits".

Mr Gove described the targeting of the base in Yavoriv, called the International Centre for Peacekeeping and Security and previously used by Nato troops to train Ukrainian soldiers, as a "significant" development. No Nato troops were at the site at the time of the attack.

While the Kremlin said the air assault killed 180 mercenaries and destroyed foreign military aid, local officials said 35 had died and 134 were injured.

A government minister said that while attacks on support convoys containing military aid were "part of war", any assault outside Ukraine's borders would be a "big moment" in the conflict.

In a weekend of fierce fighting, Russian forces were accused of using banned phosphorus munitions during an attack on Popasna, in eastern Ukraine. Phosphorus burns at more than 2,700 degrees Celsius and is almost impossible to put out once it touches the skin.

Russian soldiers were also said to have deliberately targeted an American video journalist who was killed when troops opened fire in Irpin, near Kyiv.

Brent Renaud, who had previously worked for The New York Times, was shot in the neck as he went to film fleeing refugees. It is the first reported death of a foreign journalist in the war.

Mr Sullivan told CBS News that his government was trying to establish exactly how the journalist had died before it would "execute appropriate consequences".

He said the "brazen aggression" shown by Russia was illustrated by troops targeting civilians, hospitals, places of worship "and they have targeted journalists".

Boris Johnson spoke to Mr Zelensky, condemning the killing of Mr Reynaud and the Russian capture of the mayors of Dniprorudne and Melitopol. He told Mr Zelensky: "Putin's barbaric actions are testing not just Ukraine but all of humanity."

A train carrying refugees fleeing eastern Ukraine also came under fire, killing a conductor and injuring a passenger, the country's national rail authority said.

Footage emerged of a school in the besieged southern city of Mykolaiv on fire after being hit by Russian munitions. An air strike on a monastery sheltering civilians in the east wounded 30. Ukraine's armed forces said they were launching counter attacks in Mykolaiv.

The extent of the humanitarian crisis and images of decimated homes and buildings prompted the Pope to call for an immediate ceasefire before entire cities were "reduced to cemeteries".

"In the name of God I ask you: Stop this massacre," he told crowds gathered in Rome's St Peter's Square.

The UK is to send more than 500 power generators to Ukraine to help combat the effects of widespread power cuts during a harsh winter. Mr Johnson also said Britain would discuss sending further defensive weaponry.

It came amid reports that Russia has requested military support from China as Moscow is said to be running out of some weapons. One US official told the Financial Times China was open to the idea.

Military experts on Sunday night said the training centre was targeted in an attempt to warn off other foreign fighters and stem shipments of military aid from the West into Ukraine.

It also raised the prospect that Putin considered an assault so close to a Nato ally as a strategy to bring Western leaders into top level negotiations.

The attack came hours after Sergei Ryabkov, Russia's deputy foreign minister, warned that shipments of military aid to Ukraine were "legitimate targets".

One volunteer fighter with an American or Canadian accent who was at the base told The Telegraph the bombardment "was like hell on Earth". He said he had heard that British, Irish, American and German volunteers had been trained at the base since the war started.

In recent years, the base was used by Nato allies, including soldiers from the UK, US and Canada, to train Ukrainian troops.

The joint manoeuvres were introduced after Nato launched the Partnership for Peace programme in 1994, meant to develop relationships between member states and former Soviet bloc countries.

Despite the intense fighting, both sides gave their most upbeat assessment yet of the prospects for progress at bilateral talks.

Mykhailo Podolyak, one of the Ukrainian negotiators, said it was possible the delegations could draw up draft agreement, adding: "Russia is already beginning to talk constructively. I think that we will achieve some results literally in a matter of days."