Smoke billows on the horizon as supporters of exiled Yemeni President Abedrabbo Mansour Hadi block a road in al-Mansura, east of the southern Yemeni city of Aden, as they try to make advances on the Khor Maskar area of the city held by Shiite-Houthi rebels on April 29, 2015. #

A Yemeni man grabs a child by his clothes as he searches for survivors under the rubble in houses destroyed by an overnight Saudi-led air strike on a residential area in Sanaa on May 1, 2015. #

Armed militiamen, allied to President Abedrabbo Mansour Hadi, clash with opponents and Houthi rebels in the port city of Aden's Dar Saad suburb, on April 22, 2015. #

The body of a militiaman loyal to Yemen's President Abderabbo Mansour Hadi lies in the middle of a street, during reported clashes with Houthi rebels in the port city of Aden's Dar Saad suburb on April 25, 2015. #

A Southern Popular Resistance fighter fires a weapon mounted on a truck during clashes with Houthi fighters in Yemen's southern city of Aden on May 3, 2015. Between 40-50 Arab special forces soldiers arrived in Aden on Sunday and deployed alongside local fighters against the Houthi militia, a spokesman for the Southern Popular Resistance said. #

Men on a motorbike rush a man, who was injured in crossfire between tribal fighters and Shiite rebels, to a hospital, in Taiz, Yemen, on April 26, 2015. #

Smoke billows as supporters of exiled Yemeni President Abedrabbo Mansour Hadi continue to clash with Shiite Houthi rebels in the Khor Maksar neighborhood of Aden on May 3, 2015. #

A picture taken on May 5, 2015, shows the wreckage of a Yemeni air force military transport aircraft on the tarmac of the rebel-controlled international airport of Sanaa, a day after it was destroyed by an air strike of the Saudi-led coalition. #

Displaced Yemenis shelter in a man-made underground water tunnel after their houses were destroyed by air strikes carried out by the Saudi-led alliance, in the Yemeni capital, Sanaa, on April 29, 2015. #

A Yemeni man who was wounded in an airstrike rests in an underground water tunnel where he is taking shelter with his family after their houses were destroyed by air strikes carried out by the Saudi-led alliance in Sanaa, on April 30, 2015. #

A girl rests next to her mother (not pictured) inside an underground water tunnel with other displaced Yemeni families, after they were forced to flee their home due to ongoing airstrikes in Sanaa on May 2, 2015. #

Saudi soldiers look toward the border with Yemen, at a military structure in Najran, Saudi Arabia, on April 21, 2015. A cross-border attack on Saudi Arabia by Yemeni rebel forces resulted in late-night clashes on April 30, 2015 that left three Saudi soldiers and "dozens" of Yemeni rebels dead, according to the Saudi Defense Ministry. #

Supporters of the Shiite Houthi movement brandish their weapons as they take part in a demonstration in the capital Sanaa on May 1, 2015, against the Saudi-led air campaign targeting Houthi rebels and their allies in Yemen. #

A boy looks out of the window of a Yemen Red Crescent vehicle as he leaves an underground water tunnel with other displaced people, after they were forced to flee their homes due to ongoing airstrikes in Sanaa on May 2, 2015. #

A Houthi militant holds his rifle at the yard of the residence of the military commander of the Houthi militant group, Abdullah Yahya al Hakim, after an airstrike destroyed it in Sanaa on April 28, 2015. #

A member of the Popular Resistance Committee cries as another closes the eyes of a comrade who died at a hospital of injuries he sustained during clashes with Houthi fighters in Yemen's southwestern city of Taiz on April 27, 2015. #

A man walks toward a house that was damaged during an airstrike carried out by the Saudi-led coalition in Faj Attan village, Sanaa, Yemen, on May 7, 2015. The air strikes in this part of Sanaa have forced the village's population to flee their homes. #

A worker throws a stuffed toy to another as they move furniture from a house damaged after an airstrike struck a nearby missile base, in Yemen's capital Sanaa, on April 23, 2015. #

A member of the airport security force walks in front of a destroyed Felix Airways plane, after it was hit in an airstrike, at the international airport of Yemen's capital, Sanaa, on April 29, 2015. Coalition warplanes destroyed the runway at Sanaa's rebel-held airport after an Iranian plane "defied" a blockade on Yemeni airspace, the spokesman for the Saudi-led alliance said. #

A man walks past writing on a wall along a street damaged by an airstrike in Sanaa on April 21, 2015. The writing reads, "Death to Al-Saud", referring to Saudi Arabia's royal family. #

A girl sits inside her house which was damaged by a shell which landed from a nearby missile base after the base was struck by a Saudi-led coalition air strike, near Sanaa, on April 23, 2015. #

Smoke rises after a Saudi-led airstrike hit a site where many believe the largest weapons cache in Yemen's capital, Sanaa, was located on April 20, 2015. Powerful explosions rocked the Yemeni capital early Monday morning during the strike, shattering windows and waking residents. #

A doctor examines a bullet wound on the head of Farah Abdallah, seven-year-old Yemeni refugee, at a hospital in Djibouti on May 5, 2015. Farah was shot in the head by a sniper in Aden before fleeing with her mother by boat to Djibouti. Farah has been offered financial assistance from a Yemeni benefactor to pay for her flights for critical brain surgery but no country will currently accept her. Doctors say she is at risk of death any day. #

People use a tug boat to flee Yemen's southern port city of Aden amid fighting between Houthi fighters and the Southern Popular Resistance Committees on May 5, 2015. #