Washington has been a key military backer of Ukraine, committing more than $60 billion in security aid since February 2022, but had not permitted defense contractors to work in the country after Russia launched its full-scale invasion.

"These contractors will be located far from the front lines and they will not be fighting Russian forces. They will help Ukrainian armed forces rapidly repair and maintain US-provided equipment as needed so it can be quickly returned to the front lines," the official said on condition of anonymity.

"We are taking this action because some of the equipment the US has provided Ukraine -- or will provide Ukraine in the coming months -- such as F-16s and Patriot air defense systems require specific technical expertise to maintain," the official said, adding that the decision had been made "after careful risk assessment."

Washington has provided a wide range of military equipment to Ukraine, including air defense systems, HIMARS rocket launchers, artillery pieces and munitions, as well as tanks and other vehicles.

But the election of Republican Donald Trump to a second term as president has cast doubt on the future of aid for Ukraine, and President Joe Biden's administration is seeking to provide billions of dollars in assistance before leaving office in January.