Ukraine has fired UK-made Storm Shadow missiles into Russia for the first time since the beginning of the conflict, multiple sources have told the Guardian.

The decision to approve the strikes was made in response to the deployment of more than 10,000 North Korean troops on Russia’s border with Ukraine, which UK and US officials warned was a significant escalation of the near three-year conflict.

The Guardian reported earlier this week that the UK would soon approve Storm Shadow missiles for use inside Russia after the US president, Joe Biden, agreed to do the same for the similar American Atacms weapons.

Videos uploaded to social media and circulated by pro-Russian war bloggers indicated that up to 12 missiles struck a target believed to be a command headquarters in the village of Maryno, the first confirmed use of British weapons on Russian soil during the war. Ukrainian media reported that the site may have been used by North Korean and Russian officers.

Unconfirmed images distributed via the Telegram messaging app appeared to show fragments of the missile at a location in the Kursk region. One weapons expert, Trevor Ball, formerly of the US army, said the images circulating did show Storm Shadow fragments, though he could not verify if they were current or old pictures.

There was no official confirmation from the UK, though the defence secretary, John Healey, appeared to hint at developments as he spoke to the Commons as reports of the attack in Kursk began to circulate.

Healey said he had spoken with his Ukrainian counterpart, Rustem Umerov, on Tuesday. “We’ve seen over recent weeks significant change in the action and in the rhetoric on Ukraine, and Ukraine’s action on the battlefield speaks for itself,” he said.

It marks a rapid turnaround in the UK-Ukraine relationship, following complaints that Kyiv was frustrated by a refusal by London to supply a fresh batch of Storm Shadow. The UK was said to be stung by the criticism and the complaints are partly believed to have contributed to a change of heart.

A second element was a change in policy by the outgoing Biden administration. The Storm Shadow strikes came a day after Ukraine used Atacms missiles to hit targets in the Bryansk region.

Western officials have indicated they are specifically targeting the North Korean buildup in the Kursk region, as well as infrastructure that may be used for a 50,000-strong offensive against a Ukrainian incursion there.

Storm Shadow missiles are an Anglo-French cruise missile with a maximum range of about 155 miles (250km), which so far have been used to strike Russian targets in occupied Crimea, including the naval headquarters of Russia’s Black Sea fleet. They are useful for precision targeting of bunkers and ammunition stores.

Vladimir Putin has warned that the use of US and UK-made missiles inside Russia’s borders would be tantamount to Nato entering into a direct conflict with Moscow, though western politicians have dismissed his comments as empty threats.

Western officials have warned that Russia could escalate strikes on critical infrastructure in Ukraine or use other hybrid warfare tactics against targets in Europe and other US allies around the world. British spy chiefs have accused Russia of trying to “generate mayhem” on the streets of the UK with arson and sabotage campaigns, including the secreting of incendiary devices in parcels sent via DHL, one of which caught light in a Birmingham warehouse in July, starting a small fire.

The Russian foreign minister, Sergei Lavrov, said Moscow would respond “appropriately” a day after Ukraine fired six of the newly approved Atacms missiles into an ammunition warehouse in the south-western Bryansk region.

Hours earlier, Putin signed a revised nuclear doctrine lowering the threshold for using nuclear weapons amid warnings from Russian MPs that the US action was bringing “world war three” closer.

Overnight, the Pentagon said it had seen no sign that Russia was planning to use a nuclear weapon in Ukraine, and accused Russian politicians of engaging in irresponsible rhetoric.

“We’re going to continue to monitor, but we don’t have any indications that Russia is preparing to use a nuclear weapon within Ukraine,” said the Pentagon spokesperson Sabrina Singh.

Earlier on Wednesday, the US announced it had temporarily closed its embassy in Kyiv after receiving warning of a “potential significant air attack”, advising American citizens to be prepared to move immediately to a shelter in the event of an air raid warning.

Such warnings are rare and likely to be based on specific intelligence, prompting a nervous day in Kyiv, where the mayor, Vitali Klitschko, told the public not to ignore the air raid warning that came shortly before 2pm.

City residents gathered in Kyiv’s metro stations and other locations to take shelter, and there was a heightened concern among officials, though one member of the government told the Guardian they were not aware of any particular threat.

“We don’t know exactly what could happen – it could be another major attack like Sunday, when they fired over 200 missiles and drones,” one security official said. “The Americans had some signal intelligence and were acting according to their protocol.”

The state department spokesperson, Matthew Miller, later told reporters the embassy was expected to return to normal operations on Thursday.

The US had also reportedly approved the provision of anti-personnel landmines to Ukraine.

In his nightly video address, Ukraine’s president Volodymyr Zelenskyy thanked the US and Joe Biden for the landmines which he described as “essential … to stop Russian assaults”.

Ukrainian media later on Wednesday announced that Russia had scrambled up to seven Tu-95 long-range bombers over its Engels air force base in preparation for a potential missile strike against Ukraine.

The embassies of Italy, Spain and Greece also closed temporarily on Wednesday after the US announcement.