* Army sends extra special forces to two provinces
* Widening U.S.-backed army offensive against al Qaeda
* Turmoil threatens neighbouring Saudi Arabia (Adds government report of Saudi militant killed)
By Mohammed Ghobari
SANAA, May 12 (Reuters) - An air strike killed five al Qaeda militants in Yemen on Monday, state media said, and army reinforcements were sent to two regions in an expanding offensive against Islamist insurgents posing a threat to oil-producing Saudi Arabia and the West.
The attack destroyed a car carrying the militants in the Wadi Abida district of the northern province of Maarib, a military source and a local official told Reuters. Both had earlier said it was carried out by a drone.
Yemen is waging a U.S.-backed campaign to crush Al Qaeda in the Arabian Peninsula, the global jihadi network's regional wing. AQAP has mounted dozens of attacks on government officials, security forces and foreigners in recent months.
Majed al-Mutairi, a Saudi member of AQAP, was among those killed by government forces in their two-week assault on militant positions in south Yemen, the Defence Ministry website quoted an unnamed military official source as saying on Monday.
The report of Mutairi's death follows those of several other foreign militants the Yemeni authorities say have been killed since their forces attacked Islamist strongholds in Shabwa and Abyan provinces. The report did not say when he died.
In a further sign of deteriorating security, members of the presidential guard clashed with armed tribesmen in a car near the presidential palace in Sanaa on Monday, a police source said. Residents reported hearing heavy gunfire in the area.
It was the third clash in that central part of the capital since Friday, occurring - according to police - when the tribesmen rammed their car through a checkpoint.
The previous two incidents had been blamed on Islamist militants, and appeared part of a pattern of assaults aimed at signalling their resilience in the face of the government's offensive.
Al Qaeda and its allies have grown in strength in Yemen, exploiting a security vacuum left by an uprising in 2011 that eventually overthrew autocratic president Ali Abdullah Saleh.
The chaos complicated a delicate transfer of power from Saleh to an interim government, posed a threat to neighbour Saudi Arabia and alarmed Western powers, who fear al Qaeda could launch international attacks from weakly governed Yemen.
More special forces were on their way to Maarib, east of the capital Sanaa, and the central province of al-Bayda, military sources told Reuters. The two provinces, with plenty of rugged, difficult-to-access desert and mountain terrain, offer a possible escape route for any militants fleeing the government offensive in the south.
Yemen's army says it has already killed dozens of militants since it stepped up its offensive against AQAP - regarded as one of al Qaeda's most lethal wings - two weeks ago.
A series of drone strikes in late April in al-Bayda and the southern provinces of Abyan and Shabwa killed at least 65 militants, according to state officials.
The United States has acknowledged using drones in Yemen, though it does not comment on individual strikes.
Revenge attacks blamed on Islamist militants have increased in the aftermath of the army campaign.
A suicide bomber rammed a car packed with explosives into a military police complex in the southern coastal city of Mukallah on Sunday, killing at least 11 soldiers. (Additional reporting by Mohammed Mukhashaf in Aden; Writing by Yara Bayoumy; Editing by Angus McDowall and Mark Heinrich)