* U.S. concerned about possible militant cooperation
* Rubaish says Sunni Muslims have duty to fight Shi'ites
* AQAP regarded as one of most active jihadist groups
By Mohammed Ghobari
SANAA, Aug 13 (Reuters) - A top leader of al Qaeda's Yemeni branch has hailed the seizure of swathes of Iraqi territory by the Sunni guerrilla group Islamic State, raising the possibility of cooperation between two of the most active Islamist militant networks.
A senior U.S. official last month expressed concern over possible collaboration between bombmakers from Yemen and militants in Syria, where Islamic State has also seized extensive territory in the rebellion against President Bashar al-Assad.
"I congratulate all the Mujahideen on all battlefronts and all Muslims on the victories that our brothers in Iraq have achieved against the puppets of the (Iranians)," Ibrahim al-Rubaish, the main ideologue of Al Qaeda in the Arabian Peninsula (AQAP), said in a video posted online.
"Who does not rejoice in the victory of the Sunni Muslims and the defeat of the gangs of (Iraqi Prime Minister Nuri al-)Maliki, which had tormented the Sunnis?" he added.
Islamic State in Iraq and Syria (ISIS) changed its name to Islamic State after its fighters swept through northwestern Iraq in June, and declared a caliphate spanning the parts of Iraq and Syria under its control.
ISIS originally sprang out of al Qaeda in Iraq, and shares with al Qaeda a hatred of Shi'ite Muslims, whom they regard as heretics who have started wars against Sunnis across the Middle East.
"The Sunnis of Iraq have realised that co-existence in Iraq is not possible ... and that fighting the rejectionists (Shi'ites) is a legally binding duty," said Rubaish, who is on a list of 85 people wanted by the Saudi government.
"The same situation is being repeated in Yemen and the Gulf. Will Sunnis learn the lesson of Iraq and take the initiative?"
It was not immediately possible to verify the authenticity or the date of the 11-minute recording, which appeared to have been posted on the Internet on Tuesday.
Many Sunni Islamists have hailed Islamic State's victories against a government that they regard as heavily influenced by the Shi'ite power Iran.
But most al Qaeda groups have refrained from breaking away from the parent to pledge allegiance to Islamic State leader Abu Bakr al-Baghdadi, who has declared himself caliph.
Yemeni analysts say that AQAP for its part has been trying to imitate Islamic State, which has published images of brutal killings of Shi'ite civilians and soldiers, Christians and members of other faiths.
Last week, the militant group Ansar al-Sharia, affiliated with AQAP, kidnapped 14 Yemeni government soldiers and killed them, some by slitting their throats.
Last December, militants killed medical staff in a raid on the Defence Ministry in Sanaa.
Ansar al-Sharia said it was retaliating for Yemeni army attacks and U.S. drone strikes on its strongholds in southern and eastern Yemen.
Yemeni analyst Saeed Obaid said last week that AQAP leader Nasser al-Wuhaishi was not likely to pledge allegiance to Baghdadi, but added that "he may find cooperation with ISIL, at least in secret, useful".
In May, Saudi Arabia, which borders both Yemen and Iraq, said it had uncovered a militant cell with links to both Islamic State and AQAP.
AQAP was formed in 2009 by the merger of the Yemeni and Saudi branches of the network founded by Osama bin Laden.
It is suspected of being behind a plot to blow up a U.S. airliner as it approached Detroit in 2009, and a bid the following year to place bombs hidden in computer printers on two cargo aircraft. (Reporting by Mohammed Ghobari and Sami Aboudi; Editing by Kevin Liffey)