WASHINGTON – The U.S. military is planning a new drone base in Africa that would expand its surveillance of al-Qaeda fighters and other militants in northern Mali, a development that would escalate American involvement in a fast-spreading conflict.Two Obama administration officials said military planners are eyeing the West African country of Niger as a base for unarmed Predator drones, which would greatly boost U.S. spy missions in the region.A U.S. defense official called the plan "preliminary” and said the Pentagon, the State Department, the White House and the government of Niger would all have to approve. "But it would be a good place to be, in terms of access,” the official added. The plan to locate Predator drones in West Africa was first reported Monday by the New York Times on its Web site. If approved, the plan would fill a gap in the Pentagon’s military capabilities over the Sahara, which remains beyond the reach of its drone bases in East Africa and southern Europe. U.S. officials said the plan was to use the Predators strictly for surveillance missions, not airstrikes, but they acknowledged that the drones could easily be armed if circumstances changed.The U.S. military has been flying a handful of small turboprop surveillance planes over northern Mali and West Africa for years, but the PC-12 aircraft are limited in range and lack the sophisticated sensors that Predators carry. Some senior U.S. officials have also worried that the PC-12 aircraft could be shot down by militants with a shoulder-fired missile. The U.S. ambassador to Mali, Mary Beth Leonard, suspended the flights over Mali last year because of concerns that a pilot or crew could be held hostage if forced to make an emergency landing, according to two U.S. officials who spoke on the condition of anonymity.The PC-12 turboprops have been largely based in Burkina Faso, a small West African country that shares a long border with Mali. One option under consideration at the Pentagon would be to deploy drones to Burkina Faso as well, possibly at a military base in Ouagadougou, the capital.But Niger has been gaining favor since last year, when the U.S. military relocated one of the PC-12 turboprop planes to the capital, Niamey, after reaching an agreement with Niger officials, according to a current and a former U.S. official familiar with the operation. The United States also won permission for the surveillance aircraft to refuel in the northern city of Agadez, the officials said.Army Gen. Carter F. Ham, the chief of the U.S. military’s Africa Command, visited Niger this month to discuss expanding the military relationship between the two countries, U.S. officials said.