More than 150 militants are believed to have been killed by US drone strikes in Somalia.
The large crowd of al-Shabaab fighters appeared to be "in formation" conducting "some kind of ceremony", an official told NBC News.
It's not clear if any al-Shabaab chiefs were killed in the strike.
Al-Shabaab has been responsible for terrorist attacks throughout East Africa.
It is thought the group was behind the bombing of a plane last month.
The plane was forced to make an emergency landing in Mogadishu after the explosion. The suspected bomber died but not other passengers were killed.
The group also claimed responsibility for the massacre of 67 civilians during a deadly siege at the Westgate Mall in Nairobi, Kenya in September 2013.