BUJUMBURA: Burundi on said Saturday it was taking seriously threats by Somalia's Islamist rebels to attack its capital in retaliation to this week's clashes in Mogadishu.

On Thursday, insurgents fired on Somali president's plane as he flew out, sparking an artillery exchange with Ugandan and Burundian peacekeepers that killed at least 21 and wounded dozens.

The Shebab rebels said they would attack Kampala and Bujumbura in revenge.

"Of course we take the threats by the Shebab seriously... It is something we are aware of since deploying in Somalia and we know that we have to be vigilant to protect our country," army chief Gaudefroid Niyombare told AFP.

Ugandan President Yoweri Museveni on Friday reacted defiantly to the threat.

"These terrorists, I would advise them to concentrate on their own problems. If they attack us, they will pay because we know how to attack those who attack us," he told reporters.

The African Union force in Somalia, comprising some 5,000 Burundian and Ugandan forces, denied involvement in Thursday's shooting.