An airstrike killed dozens of people Monday at a camp for displaced people in northern Yemen, in what appeared to be the single deadliest attack since a Saudi ­Arabia-led coalition sent warplanes to target Shiite insurgents advancing across the country.

As many as 40 people died and about 200 were wounded in the attack on the Mazraq camp in Hajjah province, said Joel Millman, a spokesman for the International Organization for Migration, which runs aid programs at the facility.

The Yemeni Shiite rebels, known as Houthis, accused the Saudi-led coalition of hitting the camp, located in an area under the control of the insurgents. Saudi officials did not confirm that. But, asked about the bombing, Saudi Brig. Gen. Ahmed Asiri, a coalition spokesman, asserted that the rebels were setting up positions in civilian areas and said that coalition warplanes had taken fire Monday from a residential area, forcing a “decisive response,” according to the official Saudi Press Agency.

Saudi Arabia began air attacks on Thursday to try to shore up the president of neighboring Yemen in the face of an offensive by the Houthi insurgents. At least 10 countries have joined the coalition against the rebels. The U.S. government is providing logistical and intelligence support to the operation.

The bombing highlights the risk of worsening civilian casualties in what is becoming a full-scale civil war in this impoverished Arabian Peninsula country — which hosts a formidable affiliate of al-Qaeda. Already, scores of people have been killed in fighting since the coalition operation was launched. On Monday, warships from the coalition entered the fight, pounding the southern city of Aden, residents said.

The coalition has imposed a naval blockade on Yemen’s ports, Asiri told reporters in Riyadh, the Saudi capital. He said the move is intended to intercept weapons being shipped to the Houthis — seen by Sunni Saudi Arabia as proxies of its primary foe in the region, Shiite Iran.

The Houthis, who are followers of the Zaydi sect of Shiite Islam, deny receiving weapons and training from Tehran. Residents in Yemen fear that the conflict could turn their country into a sectarian war zone, like Syria, with fighting that is fueled by Iranian-Saudi competition.

Displaced by earlier battles

The Mazraq camp is one of several U.N.-supported facilities in the north that were built to house Yemenis displaced by previous bouts of fighting between the government in Sanaa and the Houthis. Established in 2009, the camp holds about 1,100 families.

Speaking from the Saudi capital, Yemeni Foreign Minister Riyadh Yaseen blamed Houthi “artillery strikes” for killing people at the Mazraq camp.

But a senior Houthi official, Mohammed al-Bukhaiti, blamed the coalition for striking the facility, which he said demonstrated a “disregard for Yemeni blood.”

The Houthis firmly control Hajjah, and their positions in the province have been regularly targeted by the coalition, which backs embattled Yemeni President Abed Rabbo Mansour Hadi. In February, the rebels toppled his U.S.-backed government, which was an ally in fighting al-Qaeda in the Arabian Peninsula. Before fleeing Yemen last week, Hadi attempted to establish a government in Aden to rival the Houthi-controlled government in the capital, Sanaa.

The aid group Doctors Without Borders said numerous families have taken refuge in the camp in recent days because of bombings in nearby Saada province, which is the Houthis’ homeland. In a statement, the humanitarian organization said Monday’s attack killed at least 29 people, including women and children.

“People in Al Mazraq camp have been living in very harsh conditions since 2009, and now they have suffered the consequences of an airstrike on the camp,” Pablo Marco, the group’s operational manager for Yemen, said in the statement.

The Reuters news agency quoted an unidentified humanitarian worker as saying that an airstrike hit a truck carrying Houthi militiamen near the camp’s entrance.

The Yemeni Defense Ministry, which is under rebel control, also put the death toll at 40, Reuters reported.

Asiri, the coalition spokesman, emphasized that its forces were trying their best to avoid hitting civilians, according to the Saudi Press Agency.

Rebel assaults in south

The Saudi-led military campaign has not stopped the rebels from mounting assaults in the south. Over the past year, the Houthis have become Yemen’s dominant power, in part because of their prowess as hardened guerrilla fighters who have waged multiple wars against the government in Sanaa since 2004.

Attacks by the Houthis and military units loyal to former president Ali Abdullah Saleh intensified in the southern city of Aden on Monday, residents said. They reported that tanks began shelling the northern edges of the port city.

Warships from the coalition stationed off the coast began bombarding rebel positions near the city, they said.

“The fighting was intense this morning,” said Abdulnaser ­al-Arabi, 46, an anti-Houthi activist from the city. At least one coalition airstrike hit a weapons-storage facility in the Green City area of Aden, and warships struck Houthi militias as they attempted to enter the city from the eastern province of Abyan, he said.

“Those attacks appeared to have slowed the progress of the Houthis,” he said by telephone. He added that Houthi shelling damaged a medical clinic in Aden.

Saleh, who was deposed by a 2011 uprising, has used the military units that are still loyal to him to support the Houthis against Hadi and his allies.

Coalition airstrikes also pounded bases of Republican Guard forces loyal to Saleh near Sanaa early Monday. Those strikes — some of which came close to residential areas — are eliminating the air defenses and weapons-storage depots of the rebels, in what analysts say could be preparations for a possible land invasion by coalition ground troops.

Fighting also surged in other southern provinces between Houthis and local tribes who have long been their antagonists. In the province of Shabwa, heavy clashes killed dozens of Houthis and tribal fighters, said Mohammed al-Fatimi, a leader in the al-Masabein tribe.

He said that coalition warplanes targeted rebel positions in a mountainous area of the province, adding that aid also came from tribes in the neighboring province of Marib, where anti-Houthi sentiment runs high.

“The clashes are getting more intense, but the tribes stopped the Houthis and started to push them back,” Fatimi said by telephone.

Erin Cunningham contributed to this story.

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