More than 30 missiles targeted a training facility in Yavoriv, less than 15 miles from the Polish border, according to the governor of Ukraine’s western Lviv region. More than 130 people were injured.
However a source told the Daily Mirror that there were “many more killed within the site than has been claimed” and that the three men who reportedly died had not been part of a foreign fighters unit that was being trained at the International Centre for Peacekeeping and Security.
The Ministry of Defence said it could only comment on cases involving military personnel.
Asked in the aftermath of the attack, sources at the Foreign, Commonwealth and Development Office (FCDO) told The Daily Mirror that they were unaware of any British casualties among the dead.
British citizens are advised against all travel to Ukraine by the FCDO, while transport secretary Grant Shapps warned British troops and ex-service personnel last week that it is illegal for them to join the fighting in Ukraine.
Following a similar warning from defence secretary Ben Wallace, Mr Shapps told ITV’s Good Morning Britain that any Briton who does so risks worsening a “dangerous situation”.
The training centre appeared to be the most westerly target struck so far in the 18-day conflict, deepening fears of an escalation in the war should Russian missiles inadvertently strike Poland.
Nato said on Sunday that it currently does not have any personnel in Ukraine, though the US has increased the number of US troops deployed to Poland and neighbouring countries.
The Russian attack involved air-launched cruise missiles fired from Russian airspace by long-range bombers, a senior US defence official said on Monday.
The official added that there were no US troops, contractors or civilian government workers at the base.
In Moscow, defence ministry spokesperson Igor Konashenkov said Russia had used high-precision, long-range weapons to strike Yavoriv and a separate facility in the village of Starichi.
“As a result of the strike, up to 180 foreign mercenaries and a large amount of foreign weapons were destroyed,” he claimed.
The 140-square mile facility is one of Ukraine’s biggest and is the largest in the western part of the country, which has so far been spared the worst of the fighting.
The attack is reported to have spurred Poles to rush to passport offices and stockpile essentials amid fears among residents that the war could cut off supplies, or even spill over into Poland.
Downing Street said the strike so close to a Nato member was “deeply concerning” and that the UK is seeking to build the “broadest possible coalition” against the Russian aggression.
“We want everyone in the international community to condemn Putin’s barbaric acts,” Boris Johnson’s official spokesman said.
US secretary of state Antony Blinken also condemned the attack, saying: “The brutality must stop.”
Following the attack, Russia’s defence ministry warned it would take further military action against foreign fighters in Ukraine.
“At these facilities, the Kiev regime deployed a training centre for foreign mercenaries before being sent to the areas of hostilities against Russian military personnel, as well as a storage base for weapons and equipment coming from foreign countries,” it said in a statement.
“As a result of the strike, up to 180 foreign mercenaries and a large shipment of foreign weapons were destroyed. The destruction of foreign mercenaries who arrived on the territory of Ukraine will continue.”