Rival units of Yemen's divided military briefly traded fire on Thursday outside the residence of the newly elected president, Abd-Rabbu Mansour Hadi, witnesses said.

The exchange was between troops from the First Armoured Division, commanded by a general who mutinied last year against former President Ali Abdullah Saleh, and the central security force led by Saleh's nephew. There were no reported casualties.

The clash comes less than a week after Hadi was sworn in under a U.S.-backed Gulf Arab plan aimed at halting a slide into lawlessness in the impoverished Arabian peninsula state, where Islamists linked to a wing of al Qaeda have seized chunks of territory during a year of political upheaval.

Mass protests against Saleh were followed by fighting between forces loyal to him, those of rebel general Ali Mohsen and militiamen backed by tribal leaders, leaving parts of the capital in ruins and raising fears of civil war in a country on key oil shipping routes.

Saudi Arabia and Washington, both target of abortive attacks by a Yemen-based arm of al Qaeda, backed Hadi in a single-candidate election aimed at starting a two-year transition during which the military is to be restructured.

In the first significant move to shake up the military structure he inherited from Saleh, Hadi appointed a new commander for the country's politically sensitive south, state television said.

He also replaced the head of southern Aden province's security bureau and the provincial governor, the report added.

Separatist rebels have become increasingly active in the country's south - formerly a separate state and site of a 1994 civil war after four turbulent years of formal union. At least nine people killed in election day violence there.

Many southerners say they were marginalised by Saleh and his loyalists.

(Reporting by Mohammed Ghobari in Sanaa and Mohammed Mukhashaf in Aden; Writing by Joseph Logan; Editing by Alison Williams)