WASHINGTON — The United States military carried out a missile strike against a top Shabab operative in Somalia on Monday, according to Defense Department officials, three weeks after a Navy SEAL raid in another part of the country failed to capture a senior leader of the Somali Islamic militant group.

The American strike is the latest evidence that the Obama administration has decided to escalate operations against the Shabab in the aftermath of the bloody siege at a shopping mall in Nairobi, Kenya, last month in which more than 60 men, women and children were killed. A White House spokeswoman declined to comment on the strike, referring questions to the Pentagon.

Preliminary evidence collected by the military indicated that the attack killed its intended target, Ibrahim Ali, an explosives specialist for the Shabab known for his skill in building and using homemade bombs and suicide vests, a Defense Department official said.

“He’s been identified as someone we’ve been tracking for a long time,” said the official, who spoke on the condition of anonymity because the mission was conducted by the military’s secretive Joint Special Operations Command.