The US risks losing its military presence in another African country as the government of Chad sent a letter threatening to end a critical security agreement, according to four US sources, a move that threatens to cede more US influence in the region to Russia.

In a letter sent to the US defense attaché last week, Chadian officials threatened to cancel the Status of Forces Agreement, or SOFA, which determines the rules and conditions under which US military personnel can operate in the country. While the letter did not directly order the US military to leave Chad, the officials told CNN that it said all US forces would have to leave the French base in N’Djamena.