MOGADISHU (Reuters) - At least five people were killed when militiamen ambushed a convoy in central Somalia and seized heavy weapons, the local government and residents said on Tuesday, seven months after a decades-long arms embargo was lifted.

The convoy was travelling on Monday with a security escort near the town of Abudwaq when militiamen from a local clan attacked and overpowered security forces, four residents told Reuters.

They said the arms, which included machine guns, anti-aircraft weapons and rocket-propelled grenades, originated in neighbouring Ethiopia.

"It is unfortunate that five people died from both sides yesterday over weapons," said Ahmed Shire, a security adviser to the president of Galmudug State, where Abudwaq is located. "We understand the weapons fell into the hands of civilians."

Rashid Abdi, an analyst with the Sahan Research think-tank, called the incident "the single most serious incident of arms proliferation in central Somalia" and said some of the weapons would likely be bought by al Shabaab, an al Qaeda affiliate.

Writing on X, Abdi said it was a "clear case of why lifting of arms embargo on Somalia was big strategic mistake".

The last restrictions under an arms embargo that had been in place, in some form, for more than 30 years were fully lifted by the U.N. Security Council in December.

Somalia's government said the move would allow it to confront security threats and build up its national forces.

The country has been confronting an insurgency by al Shabaab for nearly two decades. African Union peacekeepers are drawing down their presence to hand over great responsibility to Somali forces, but the AU and Mogadishu government have warned of a possible security vacuum if that happens too quickly.

Clan-based militias have fought alongside government forces in a military campaign against al Shabaab in central Somalia. They have also turned their weapons on one another in disputes over land and resources.

(Reporting by Abdi Sheikh in Mogadishu and Aaron Ross in Nairobi; Editing by Alison Williams)

By Abdi Sheikh and Aaron Ross