A US drone strike on an Al Shabaab training camp in Somalia is believed to have killed 150 militants, according to the Pentagon.

"The fighters were there training and were training for a large-scale attack. We know they were going to be departing the camp and they posed an imminent threat to US and (African Union) forces," Pentagon spokesman Captain Jeff Davis said.

"Initial assessments are that more than 150 terrorist fighters were eliminated," he added.

The camp, about 120 miles (195km) north of the capital Mogadishu, was hit on Saturday.

The Al Qaeda-linked group is battling an African Union military force supporting a Western-backed government.

Earlier on Monday, an AU peacekeeper and two Somali soldiers were wounded when Al Shabaab detonated a bomb outside the airport in the town of Beledweyne.

"We were behind the blast that targeted Djibouti forces. We shall give details of casualties later," Sheikh Abdiasis Abu Musab, Al Shabaab's military operation spokesman, told Reuters.

The bomb was hidden in a "paper bag", according to Ibrahim, a local military officer who did not wish to reveal his full name.

Last month, a plane was forced to make an emergency landing after a bomb smuggled aboard at Mogadishu International Airport exploded mid-air, killing the suspected suicide bomber.

Meanwhile, a huge cache of weapons bound for Somalia has been seized off the coast of Oman.

The Australian navy, which patrols waters around the Indian Ocean as part of an international maritime force, said it seized nearly 2,000 AK-47 rifles, 100 rocket-propelled grenade launchers, 49 PKM machine guns, 39 PKM spare barrels and mortar tubes from the fishing vessel.

It was not immediately clear who the weapons were being delivered to.