A group of Islamist fighters in Yemen renounced their loyalty to al Qaeda's leader and pledged allegiance to the head of the Islamic State, according to a Twitter message retrieved by U.S.-based monitoring group SITE.
The monitoring group could not immediately verify the statement distributed on Twitter purportedly from supporters of al Qaeda in the Arabian Peninsula (AQAP) based in central Yemen.
AQAP is considered the most powerful branch of the global militant network headed by Ayman al-Zawahiri and has previously rejected the authority of Islamic State which has declared a caliphate, or Islamic theocracy, in swathes of Iraq and Syria.
State authority in Yemen has unravelled since a Shi'ite Muslim militia formally seized power last week and the Sunni AQAP has sworn to destroy it, stoking fears of sectarian civil war.
"We announce the formation of armed brigades specialized in pounding the apostates in Sanaa and Dhamar," the purported former AQAP supporters wrote, referring to two central provinces.
"We announce breaking the pledge of allegiance to the sheikh, the holy warrior and scholar Sheikh Ayman al-Zawahiri ... We pledge to the caliph of the believers Ibrahim bin Awad al-Baghdadi to listen and obey," they said.
Militants in Egypt's Sinai Peninsula and Libya have also joined Islamic State, signalling a competition for loyalty among armed Islamists battling states in the Mideast and North Africa.