U.S. President Donald Trump signs the "National Defense Authorization Act for Fiscal Year 2020" at Joint Base Andrews, Maryland, U.S. December 20, 2019. Reuters-Yonhap

U.S. President Donald Trump on Friday signed a defense policy bill containing provisions restricting the drawdown of American troops in South Korea.

The National Defense Authorization Act (NDAA) for Fiscal Year 2020, which authorizes funding for the Department of Defense, passed the Senate on Tuesday and was sent to Trump to be enacted into law.

The U.S. president signed the bill during a ceremony held at Joint Base Andrews, Maryland, before departing for Florida for the Christmas holidays.

The new legislation prohibits the use of funds to reduce the number of American troops stationed in South Korea below the current level of 28,500 unless the U.S. defense secretary certifies that it is in the U.S. national security interest.

The defense secretary must also declare that the reduction will not significantly undermine the security of U.S. allies in the region, and that allies including South Korea and Japan have been appropriately consulted.