WASHINGTON (Reuters) - The United States carried out a drone strike in Somalia against a senior leader of militant group al Shabab, the Pentagon said on Tuesday, adding his death would represent a major blow to a group already losing ground in the country.

The Jan. 31 Hellfire missile strike south of the capital Mogadishu targeted Yusef Dheeq, the group's chief of external operations and planning for intelligence and security, Pentagon spokesman Rear Admiral John Kirby said.

"I'm not in a position now to confirm the results of the strike, but if successful – if he no longer breathes - then this is ... another significant blow to al Shabab," Kirby said.

The United States did not believe there were any civilian casualties, he added.

The strike was the latest in an ongoing campaign against al Shabab, whose leadership is affiliated with al Qaeda. In September, a U.S. drone strike killed the group's main leader, Ahmed Abdi Godane. Another drone strike on Dec. 29 killed another senior al Shabab militant.

Al Shabab, which controlled Mogadishu and southern Somalia until it was driven out of the capital in 2011, has steadily lost territory to a military offensive by Somali and African Union forces.

The group still launches guerrilla attacks and bombings and was behind a 2013 raid on a Nairobi shopping mall that killed 67 people.