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

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 in what UK and US officials have warned was a major escalation of the nearly three-year-old conflict.

The Guardian earlier this week reported 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 weapon.

It was not immediately clear what Ukraine used the Storm Shadow missiles to target. Unconfirmed images distributed via the Telegram messaging app appeared to show fragments of the missile at a location with 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 verify if they were current or old pictures.

The strike came one day after Ukraine used US-supplied missiles to strike targets in the Bryansk region. Western officials have indicated that they’re specifically targeting the North Korean buildup in the region as well as infrastructure that may be used for a 50,000-strong offensive against a Ukrainian incursion into the region.

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.