WASHINGTON — The escalating American military engagement in Somalia has led the Obama administration to expand the legal scope of the war against Al Qaeda, a move that will strengthen President-elect Donald J. Trump’s authority to combat thousands of Islamist fighters in the chaotic Horn of Africa nation.

The administration has decided to deem the Shabab, the Islamist militant group in Somalia, to be part of the armed conflict that Congress authorized against the perpetrators of the Sept. 11, 2001, terrorist attacks, according to senior American officials. The move is intended to shore up the legal basis for an intensifying campaign of airstrikes and other counterterrorism operations, carried out largely in support of African Union and Somali government forces.

The executive branch’s stretching of the 2001 war authorization against the original Al Qaeda to cover other Islamist groups in countries far from Afghanistan — even ones, like the Shabab, that did not exist at the time — has prompted recurring objections from some legal and foreign policy experts.

The Shabab decision is expected to be publicly disclosed next month in a letter to Congress listing global deployments. It is part of the Obama administration’s pattern of relaxing various self-imposed rules for airstrikes against Islamist militants as it tries to help its partner forces in several conflicts.