LONDON (AP) — The U.S. Air Force says a number of small drones were detected last week around three bases in eastern England that are used by American forces.

The drones were spotted between Wednesday and Friday near RAF Lakenheath, RAF Mildenhall and RAF Feltwell. They were actively monitored after they were seen in the vicinity of and over the three bases, U.S. Air Forces Europe said in a statement.

The Air Force didn’t identify who was behind the incursions, but said base officials determined there was no impact on residents or critical infrastructure.

Lakenheath is home to the 48th Fighter Wing, which the U.S. Air Force describes as the foundation of its combat capability in Europe. Mildenhall hosts the 100th Air Refueling Wing, and Feltwell is a hub for housing, schools and other services.

“To protect operational security, we do not discuss our specific force protection measures but retain the right to protect the installation,” the Air Force said. “We continue to monitor our airspace and are working with host-nation authorities and mission partners to ensure the safety of base personnel, facilities and assets.”

While it is unclear whether the drones had hostile intent, the incidents came during a week that saw the most significant escalation of hostilities in Ukraine since Russia’s full-scale invasion nearly three years ago.

For the first time, Ukraine struck targets inside Russia with intermediate range missiles supplied by the U.S. and Britain after President Joe Biden and Prime Minister Keir Starmer authorized use of the weapons.

In response, Russia launched a new intermediate range ballistic missile at Ukraine, and President Vladimir Putin said his country had the right to strike nations that allow their weapons to be used against Russia.

Lakenheath, Mildenhall and Feltwell, located close to one another in the counties of Suffolk and Norfolk, are Royal Air Force bases used primarily by the U.S. Air Force.

Britain’s Ministry of Defense said “we take threats seriously and maintain robust measures” at military installations.

“This includes counter drone security capabilities. We won’t comment further on security procedures,” it said.