Suicide bombers attacked two mosques linked to Yemen’s powerful Shiite rebels Friday, killing scores of worshipers and wounding hundreds more in a further sign that the country is collapsing into sectarian chaos.

The attacks come a day after intense clashes in the southern city of Aden and an attempted assault on an oil-rich province by the rebels, known as Houthis.

An increasingly unstable Yemen poses security concerns for the United States, given that the country has been a staging ground for attacks by Yemen’s branch of al-Qaeda against U.S. military and other Western targets.

The bombers targeted mosques used mainly by supporters of the Houthis, who have captured vast territory in a military assault that many Yemenis fear is turning the country into a proxy battleground for regional powers. Shiite Iran has boosted support for the Houthis, while Sunni Saudi Arabia backs their enemies.

Shortly after the attacks in the capital, Yemeni medical workers put the death toll at 46 and the number of wounded at 100. By late afternoon, a Houthi-run television network said there were 137 dead and nearly 350 wounded.

“There were two explosions that happened in the mosque: one in the front next to where I was sitting and another in the back. Both were carried out by suicide bombers,” said Abdullah Abdulkarim al-Houthi, a 50-year-old Houthi official who sustained shrapnel wounds in his arms and legs.

He added, “Dr. al-Muhatwari didn’t survive.”

Hashem Abdullah, 28, was standing near the other mosque that was attacked, known as al-Hashoosh. He said a car bomb exploded next to the building just before a suicide bomber entered and detonated his explosives. The incident occurred shortly after the end of Friday prayers, when the mosque was teeming with people, he said.

“The explosion caused confusion among all the people, and soon after that a suicide bomber ran into the mosque and blew himself up near the area of the imam of the mosque,” said Abdullah, referring to the prayer leader.

Abdullah said he spent part of the afternoon clearing away charred Korans and clothing and loading trucks with the human remains from the attacks at Hashoosh and the nearby Badr mosque, which was targeted at the same time.

After the attacks, Houthi militiamen fanned out across the area where the bombings took place, known as Juraf, blocking roads and inspecting civilians for weapons. Several Toyota pickup tricks with machine guns mounted on top blocked roads.

Footage aired by the Houthi television channel al-Masirah showed emergency responders frantically carrying bodies in blood-soaked sheets. The bodies were loaded onto pickup trucks while men in the background shouted in grief.

A group calling itself the Sanaa Province and claiming affiliation with the Islamic State claimed responsibility for the attacks. In a message transmitted via Twitter, it warned that the bombings were the “tip of the iceberg that is coming,” according to a translation by the SITE Intelligence Group.

An Islamic State claim of responsibility for the attacks was greeted with some skepticism among U.S. officials, who noted that the group is not known to have a significant presence in Yemen. By contrast, other groups, including al-Qaeda in the Arabian Peninsula (AQAP), have had a foothold in the Yemeni capital for years and have carried out previous attacks.

AQAP, however, released a statement Friday denying responsibility for the bombings, according to the SITE group.

White House press secretary Josh Earnest said Friday that the United States is investigating the claim of responsibility by the Yemeni branch of the Islamic State and is trying to determine whether the group has the organizational depth to coordinate such attacks, according to an Associated Press report.

Earnest said the Islamic State often claims responsibility for attacks purely for propaganda value.

U.S. officials said that even if the bombers were not dispatched or directed by the Islamic State, militants eager to associate themselves with the group’s brand may be behind the bombings.

The Islamic State’s claim reflects an apparent new determination by the group to take propaganda advantage of distant attacks and depict itself as expanding elsewhere even as it struggles to maintain its grip on territory in Iraq. The group alos has recently accepted a pledge of loyalty from Boko Haram in Nigeria and asserted responsibility for a deadly assault Wednesday on a museum popular with Western visitors in Tunisia.

The latter claim is seen as more plausible, U.S. officials said, because thousands of Tunisian citizens have traveled to fight in Syria, where many have likely formed ties to the Islamic State.

Mohammed al-Bukhaiti, a Houthi official, lashed out at the West and Sunni Arab countries, accusing them of colluding with AQAP in Friday’s attacks.

“They all move as one team, and they are supported by the United States and Britain, and this alliance is funded by Saudi Arabia and Qatar,” he said by telephone.

The mosque attacks follow intense fighting in the southern city of Aden between forces loyal to President Abed Rabbo Mansour Hadi and military units thought to be under the control of Ali Abdullah Saleh, who was ousted as president during an uprising in 2011.

Also Friday, Yemeni security officials said that AQAP took control of a southern city after security forces there surrendered. The AP reported that the officials said al-Qaeda militants driving pickup trucks and flying black flags swept through the city of Houta, the capital of Lahj province. They took over the main security barracks, the governor’s office and the intelligence headquarters, which houses prisons holding al-Qaeda detainees.

The officials said security ­forces loyal to Saleh surrendered to the militants without resistance. Those who resisted at the governor’s office were summarily killed, the officials said, speaking on condition of anonymity because they were not authorized to talk to the media.

On Thursday, Houthi militants attempted to attack the oil-rich province of Marib but were repelled by tribal forces, Yemeni officials said.

Marib is considered strategically vital because of its oil and gas reserves, and the area’s power plants provide electricity to Sanaa and other parts of Yemen. The largely Sunni tribes of Marib have vowed to fight Houthi incursions, and Shiite rebels, diplomats and analysts say that Saudi Arabia has substantially increased funding to the tribesmen to defend the area.

Saudi Arabia and other Sunni Arab countries have backed the governing authority in Aden that Hadi established in opposition to the Houthi-controlled government in Sanaa. One Yemeni official from the area said the Saudis have given significant funding to the president to win support from tribes in southern Yemen. The official spoke on the condition of anonymity, citing concern for his safety.

In January, the Houthis put Hadi under house arrest in Sanaa and compelled him to resign. Last month, he fled Houthi captivity for Aden, rescinded his resignation and asserted that he was the legitimate leader of the country.

Naylor reported from Beirut. William Branigin and Greg Miller in Washington contributed to this report.

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