Shi'ite Houthi rebels man a checkpoint at the southern entrance to the city of Sanaa November 15, 2014. (Photo by Mohamed al-Sayaghi/Reuters)

People walk past a graffiti reading “Al Qaeda is American-made” at the site of a recent car bomb attack in Sanaa January 14, 2015. (Photo by Khaled Abdullah/Reuters)

Mourners carry the coffin of a soldier, who was killed by al Qaeda militants in the Wadi Hadramout region in northeastern Yemen, during his funeral in Sanaa August 10, 2014. (Photo by Khaled Abdullah/Reuters)

Burnt-out trees lie in the yard of a house near the residence of the Iranian ambassador after a bomb attack in Sanaa December 3, 2014. (Photo by Khaled Abdullah/Reuters)

Defendants look out from behind bars as they stand in the holding cell of the state security court in Sanaa December 2, 2014. (Photo by Khaled Abdullah/Reuters)

A poster with an image of a soldier lies on his grave after he was killed by al Qaeda militants in the Wadi Hadramout region in northeastern Yemen, at a military cemetery in Sanaa August 10, 2014. (Photo by Khaled Abdullah/Reuters)

A notebook belonging to an al Qaeda fighter, which was found in a former militant training camp in southern Yemen in May 2014, is seen in this picture taken July 8, 2014. The left page shows diagrams depicting various types of ambushes. (Photo by Martin Dokoupil/Reuters)

People walk past graves which were dug up for soldiers killed in the Wadi Hadramout region in northeastern Yemen, at a military cemetery in Sanaa August 10, 2014. (Photo by Khaled Abdullah/Reuters)

Soldiers stand at a post which was previously controlled by al Qaeda insurgents in al-Mahfad, in the southern Yemeni province of Abyan May 23, 2014. (Photo by Khaled Abdullah/Reuters)

A girl is seen in her family's flat damaged during a police raid on a nearby hideout of al Qaeda militants in the Arhab region north of the Yemeni capital Sanaa May 27, 2014. (Photo by Khaled Abdullah/Reuters)

Military men wave their weapons at an army post at al-Mahfad, in the southern Yemeni province of Abyan May 23, 2014. (Photo by Khaled Abdullah/Reuters)

Boys look at a vehicle destroyed during a police raid on a hideout of al Qaeda militants in the Arhab region, north of the Yemeni capital Sanaa May 27, 2014. (Photo by Khaled Abdullah/Reuters)

An army officer stands on the ruins of a police barracks, which was bombed by al Qaeda insurgents, at al-Mahfad, in the southern Yemeni province of Abyan May 23, 2014. (Photo by Khaled Abdullah/Reuters)

Tribesmen loyal to the Shi'ite Houthi group attend a tribal gathering to show support to the group in Amran province north of the Yemeni capital Sanaa March 13, 2014. (Photo by Khaled Abdullah/Reuters)

The body of a man is seen tied to a soccer goal post after al Qaeda militants shot him to death, accusing him of spying for the United States outside al-Shihr city of the southeastern Yemeni province of Hadhramout March 6, 2014. (Photo by Reuters/Stringer)

A boy looks at the wreckage of a car hit by an air strike in the central Yemeni province of al-Bayda April 19, 2014. (Photo by Reuters/Stringer)

Policemen stand guard next to a wall of the central prison in Sanaa after a bomb exploded outside, February 14, 2014. (Photo by Mohamed al-Sayaghi/Reuters)

Honour guards carry the coffins of victims of an attack on the defence ministry during a funeral procession in Sanaa December 9, 2013. (Photo by Khaled Abdullah/Reuters)

A suspected al Qaeda militant reacts from behind bars as his verdict is being pronounced at the state security court of appeals in Sanaa March 26, 2013. (Photo by Khaled Abdullah/Reuters)

People gather outside a house destroyed by an air strike on a suspected hideout of al Qaeda militants in al-Shiher city of Yemen's southeastern province of Hadramount November 22, 2013. (Photo by Reuters/Stringer)

Yemeni soldiers stand on the ruins of a machine gun mounted on a truck at the site of a police barracks, which was bombed by al Qaeda insurgents, in al-Mahfad in the southern Yemeni province of Abyan May 23, 2014. (Photo by Khaled Abdullah/Reuters)

Boys gather near the wreckage of car destroyed last year by a U.S. drone air strike targeting suspected al Qaeda militants in Azan of the southeastern Yemeni province of Shabwa February 3, 2013. (Photo by Khaled Abdullah/Reuters)

People gather near the wreckage of a car destroyed by a U.S. drone air strike that targeted suspected al Qaeda militants in August 2012, in the al-Qatn district of the southeastern Yemeni province of Hadhramout February 5, 2013. (Photo by Khaled Abdullah/Reuters)

Boys walk in a house destroyed by an air strike last year that was targeting al Qaeda-linked militants, in the southern Yemeni town of Jaar February 1, 2013. (Photo by Khaled Abdullah/Reuters)

Children play at a school sheltering them in the southern Yemeni port city of Aden October 8, 2012. (Photo by Khaled Abdullah/Reuters)

The body of a suspected al Qaeda militant lies in a house raided by security forces in the southern Yemeni port city of Aden October 2, 2012. (Photo by Reuters/Stringer)

An army soldier stands near a building destroyed during recent fighting between the army and al Qaeda-linked militants in the southern Yemeni city of Zinjibar June 21, 2012. (Photo by Khaled Abdullah/Reuters)

Members of Yemen's elite Republican Guard and pro-army tribesmen gather for a group photo atop a military vehicle as they secure a road leading to Lawdar town in the southern province of Abyan June 19, 2012. (Photo by Khaled Abdullah/Reuters)

A Yemeni army officer looks out from a tank while on a road leading to the southern Yemeni city of Zinjibar June 14, 2012. (Photo by Khaled Abdullah/Reuters)

A view shows military vehicles destroyed during recent fighting between the Yemeni army and al Qaeda-linked militants in the southern Yemeni city of Zinjibar June 14, 2012. (Photo by Khaled Abdullah/Reuters)

Yemeni army soldiers fire at positions of al Qaeda-linked militants in the southern Yemeni town of Shaqra June 14, 2012. (Photo by Reuters/Yemen's Defence Ministry)

An army soldier stands guard at a local authority compound damaged during recent fighting between the army and al Qaeda-linked militants in southern Yemeni city of Zinjibar June 14, 2012. (Photo by Khaled Abdullah/Reuters)

An army tank is driven on a road near the southern Yemeni city of Zinjibar June 14, 2012. (Photo by Khaled Abdullah/Reuters)

Vehicles and buildings destroyed during recent fighting between the army and al Qaeda-linked militants are seen on a road leading to the southern Yemeni city of Zinjibar June 14, 2012. (Photo by Khaled Abdullah/Reuters)

A vehicle belonging to al Qaeda-linked militants and destroyed during clashes with the army, is seen at the front line in the southern province of Abyan June 4, 2012. (Photo by Reuters/Yemen's Defence Ministry)

Yemeni army forces fire a missile towards of al Qaeda-linked militants positions in the southern province of Abyan June 4, 2012. (Photo by Reuters/Yemen's Defence Ministry)

An army tank fires during a firefight against militants linked to al Qaeda near the southern Yemeni city of Zinjibar May 30, 2012. (Photo by Reuters/Yemen's Defence Ministry)

A boy holds a candle during a candlelight vigil to condemn a suicide attack that killed over 100 soldiers in Sanaa May 28, 2012. (Photo by Mohamed al-Sayaghi/Reuters)

A soldier looks at portraits of victims of a suicide attack, during a protest to condemn the attack that killed more than 90 soldiers, in Sanaa May 29, 2012. (Photo by Mohamed al-Sayaghi/Reuters)