CAIRO (AP) — The U.S. military says it has lost an unmanned drone aircraft over the Libyan capital, Tripoli.

Rival armed groups have been fighting there for control of the city since April.

The U.S. Africa Command said late Friday that the drone was lost a day earlier while assessing the security situation and monitoring extremist activity.

AFRICOM did not give a reason for the drone’s loss, but said it was investigating.

In September, the U.S. military said it carried out several airstrikes against the Islamic State group in Libya, killing 40 militants. Those were the first U.S. strikes in the North African country in over a year.

Oil-rich Libya remains fractured after descending into chaos in 2011, when an international military coalition helped rebels overthrow longtime autocrat Moammar Gadhafi.