By Mohammad Mukhashaf and Sami Aboudi
ADEN, March 30 (Reuters) - An air strike killed at least 40 people at a camp for displaced people in north Yemen on Monday, humanitarian workers said, in an attack which apparently targeted a nearby base for Houthi fighters battling President Abd-Rabbu Mansour Hadi.
Yemen's state news agency Saba, which is under the control of the Houthis, said the camp at Haradh was hit by Saudi planes. It said the dead included women and children, and showed the bodies of five children laid out on a blood-streaked floor.
Hadi's Foreign Minister Riyadh Yassin blamed Houthi artillery for the explosion at the camp near Haradh. Saudi military officials were not immediately available for comment.
The International Organization for Migration, which initially reported 45 deaths, said 40 people were killed and 200 wounded - dozens of them severely hurt.
A humanitarian worker said earlier that the strike hit a truck of Houthi militiamen at the gate to the Mazraq camp, killing nine camp residents, two guards and an unknown number of fighters.
Mazraq, in the province of Hajja next to the Saudi border, is a cluster of camps home to thousands of Yemenis displaced by over a decade of wars between the Houthis and the Yemeni state, as well as East African migrants.
Saudi Arabia, supported by regional Sunni Muslim allies, launched an air campaign to support Hadi after he withdrew last week from the capital to Aden. He left Yemen on Thursday to attend an Arab summit and has not returned.
The fighting has brought civil war to the Arabian Peninsula's poorest country, which was already sliding into chaos with a growing southern secessionist movement and a covert U.S. drone campaign - now stalled - against al Qaeda in the east.
The growing power of the Houthis, part of a Shi'ite minority that makes up about a third of the population, also means Yemen has become the latest stage for Saudi Arabia's power struggle with Iran.
The two regional rivals support opposing sides in Syria's civil war and in neighboring Lebanon. Tehran also supports and arms Shi'ite militias in Iraq, although it denies Riyadh's accusations that it supports Yemen's Houthis militarily.
WARSHIPS FIRE ON HOUTHIS
In the capital Sanaa, controlled by the Houthis, jets struck around the presidential palace overnight and made more raids throughout the day. Most of the air strikes, launched on Thursday, have taken place so far only at night.
In the south, Houthi fighters closed in on the port city of Aden, the last major stronghold of Hadi supporters, and residents said Egyptian warships shelled a column of Houthis advancing along the coastal road.
It was the first known report of naval forces taking part in the conflict. A Reuters reporter heard heavy explosions and saw a thick column of black smoke rising from the area about 15 km northeast of Aden and witnesses reported at least one plane flying nearby.
While Hadi's fighters ceded ground around Aden, Pakistan announced it would send troops to support the Saudi-led coalition.
"We have already pledged full support to Saudi Arabia in its operation against rebels and will join the coalition," a Pakistani official said.
In a cabinet statement, Saudi King Salman said his country was open to a meeting of all Yemeni political parties willing to preserve Yemen's security, under the auspices of the six-nation Gulf Cooperation Council, most of which are part of Riyadh's anti-Houthi coalition.
The Arab leaders agreed at their meeting in Egypt to form a unified military force to counter growing regional security threats such as the Yemen conflict.
But working out the logistics of the force will be a protracted process and Yemen's rugged geography, internal power struggles and recent history all present challenges to any military campaign.
Just four years after the 1990 unification of North and South Yemen, civil war erupted when southerners tried to break away, but were defeated by Saleh's northern forces.
In the 1960s, intervention by Saudi Arabia and Egypt on opposing sides of a civil war in North Yemen led to a long and damaging military stalemate.
Saudi Arabia says it is focusing for now on air strikes against the Houthis, rather than a ground campaign, promising to increase pressure on them over coming days.
On Sunday, sources said Yemeni exports of liquefied natural gas (LNG) were running as normal despite the shutdown of major seaports. But French oil firm Total said on Monday operations at its Block 10 had been reduced, with gas production maintained only for local power generation and to supply nearby areas.
Several countries have evacuated citizens from Yemen in recent days. About 500 Pakistani nationals were flown out of the Red Sea port of Hodeida on Sunday, and India said on Monday it was preparing to fly out 500 people from Sanaa.
Saleh was forced to step down after a 2011 popular uprising, ending more than three decades in power, but still wields influence and has the loyalty of much of the army.
(Additional reporting by William Maclean, Noah Browning and Rania El Gamal in Dubai, Angus McDowall in Riyadh, Stephanie Nebehay in Lausanne; Writing by Dominic Evans; Editing by Angus MacSwan)