The U.S. military will divest an air base in Britain and return it and 14 other facilities in Europe back to their home nations, the Pentagon said on Thursday, in a bid to save the U.S. government about 500 million U.S. dollars annually.

The restructuring will pull thousands of U.S. military and civilian personnel out of the 15 sites mostly in the United Kingdom and Germany over several years, the Pentagon said in a statement.

The Pentagon would divest RAF Mildenhall, northeast of London, and withdraw 3,200 U.S. personnel from the base home to tanker, reconnaissance and special operations aircraft over the next several years, it said.

The Pentagon also has selected RAF Lakenheath in the UK to be the first permanent European base for the F-35 Joint Strike Fighter. The first aircraft are scheduled to arrive in the UK in 2020 to replace F-15 fighter jets, it said. "The decision reflects the closeness of the U.S.-UK defense relationship and the military value of the basing in the UK," added said.

The reduction in RAF Mildenhall would be partially offset as about 1,200 U.S. military personnel would be permanently stationed at RAF Lakenheath along with two squadrons of F-35 fighters, the Pentagon said, adding that the net loss of U.S. troops in Britain would be about 2,000.

About nine facilities in Germany would be closed or partially closed, but overall U.S. troop numbers in the country were expected to increase a few hundred, according to the statement. In Portugal, some 500 U.S. military and civilian personnel would be withdrawn from Lajes Field in the Azores.

"These actions will result in a slight decrease in our force levels, but will also ensure they are most effectively positioned to enable the U.S. and our NATO allies to respond quickly to crises worldwide," the statement said.

"Additionally, these actions will result in the greater efficiency and effectiveness of our presence in Europe by reducing the U.S. footprint while ensuring that the infrastructure in place will continue -- now and into the future -- to sustain U.S. interests and support our partners in the region," it said.

The transformation of the infrastructure will help maximize U.S. military capabilities in Europe and help strengthen America's important European partnerships, U.S. Defense Secretary Chuck Hagel said in the statement.

The Pentagon has been systematically taking U.S. forces out of Europe in recent years, reflecting the ongoing decrease in the size of the Army and Marine Corps as well as the increased emphasis on the Pacific and a desire to shift additional troops into Eastern Europe and other regions where tensions with Russia have grown.