A French commando raid in Somalia to free intelligence agent Denis Allex ended in the deaths of 17 Islamists and a French soldier. France said the hostage also died in the failed rescue, but the man's captors denied he had been killed. (Anonymous/AP)

U.S. military fighter jets provided backup support to a failed French hostage rescue mission in Somalia, the White House announced Sunday in a rare public acknowledgment of American combat operations in the Horn of Africa.

In a letter to Congress, President Obama said U.S. combat aircraft “provided limited technical support” to French forces late Friday as they attempted to rescue a French spy who had been held captive for more than three years.

The risky mission by French commandos ended disastrously after a gun battle with Islamist fighters from the al-Shabab militant network. The hostage, identified by his cover name, Denis Allex, was presumed killed and a French soldier was reported missing.

Obama said the U.S. warplanes “briefly” entered Somali airspace but did not open fire and departed Somalia by 8 p.m. Friday, Washington time. He said he approved the mission but gave no other details.

A U.S. defense official, speaking on the condition of anonymity to discuss the operation, said the combat aircraft were based at Camp Lemonnier in Djibouti, a small country on Somalia’s northwestern border.

The U.S. military has based a growing number of armed Predator drones as well as F-15 fighter jets at Camp Lemonnier, which has grown into a key installation for secret counterterrorism operations in Somalia and Yemen. The defense official declined to identify the aircraft used in the rescue attempt but said they were fighter jets, not drones.

The White House said Obama was obligated under the War Powers Act to notify Congress within 48 hours of the operation because U.S. military aircraft entered another country in support of a combat mission.

It was unclear, however, why Obama felt compelled to reveal this particular operation when he has remained silent about other specific U.S. combat missions in Somalia. Spokesmen from the White House and the Pentagon declined to elaborate or answer questions Sunday night.

U.S. drones regularly conduct surveillance flights over Somalia and occasionally launch airstrikes against suspected members of al-Shabab. U.S. Special Operations Forces have also entered Somalia in the past, including a hostage rescue mission in January 2012.

In a separate letter to Congress in June, Obama acknowledged for the first time that the U.S. military had engaged in “direct action” against al-Shabab fighters in Somalia and al-Qaeda militants in Yemen. That notification to lawmakers was also made under the War Powers Act, but most details about the operations were kept classified.

Scott Wilson contributed to this report.