SANA, Yemen — In the deadliest attack on civilians in Yemen in recent memory, more than 100 people were killed on Friday when suicide bombers attacked two Zaydi Shiite mosques in the capital, Sana, during weekly prayers. A group claiming to be the Yemeni division of the Islamic State militant group said it was responsible for the attack, raising fears of a growing shift toward sectarian violence in the country’s civil conflict.
Hospitals in the capital made urgent appeals for blood to treat the hundreds of people injured in the explosions at the Badr and Hashoush mosques, which were apparently coordinated. Another suicide bomber was detected before he could reach a mosque in the northern province of Saada, a stronghold of the Houthi rebel movement, which controls Sana and since September has been Yemen’s most dominant force.
An Interior Ministry official said at least 60 people were killed at each mosque, but the death toll is expected to rise.The most recent attack on civilians in the capital was in January, when a car bomb killed more than 30 people outside a police academy.
Sunni extremists, including the Islamic State fighters and militants linked to an affiliate of Al Qaeda in Yemen, have carried out a number of deadly attacks against supporters of the Houthis, whose leaders are members of the Zaydi branch of Shiite Islam and are considered heretics by the Sunni militants.
But bombings of mosques have been rare, and there are increasing fears that a more radical force is intervening in Yemen’s conflict. Suspicion quickly turned to the Islamic State, which has seized territory in Iraq and Syria and spawned affiliates in Libya and Egypt.
The carnage on Friday came after days of fighting across Yemen, marking a violent new stage in a seven-month-old political crisis that is increasingly taking on the character of a civil war.
Yemen has been leaderless since January, when the Houthis tightened their grip on the capital and placed the president, Abdu Rabbu Mansour Hadi,along with his government, under house arrest.
Mr. Hadi later fled to the southern port city of Aden and declared that he was still the country’s leader, splitting the country between competing centers of power. Diplomats, including those at the United Nations, have been unable to broker a compromise, as many regional powers, including Iran and Saudi Arabia, seem to have abandoned the effort, throwing their support behind either Mr. Hadi or the Houthis and inflaming the conflict.
Earlier this week, unidentified assassins shot and killed one of Yemen’s most prominent dissident journalists and a supporter of the Houthis, Abdulkarim al-Khaiwani, outside his home in Sana. On Thursday, violence spread to Aden in a day of rare factional clashes over control of the international airport and a security base.
The fighting in Aden pitted tribesmen and military units loyal to Mr. Hadi against a security unit seen as close to Yemen’s former president, Ali Abdullah Saleh, who has remained influential in the country and allied himself with the Houthis.
There were more reports of fighting in the south on Friday. Warplanes bombed Mr. Hadi’s presidential compound in Aden for a second day, but did not cause any casualties, according to witnesses and security officials in the city.
Blood could be seen on the street outside the Badr mosque, where the bombers maximized the number of casualties by detonating their explosives inside but also among the overflow of worshipers outside. Witnesses said 12 members of one family were killed.
Two suicide bombers also attacked the Hashoush mosque, with one hiding his explosives in a fake cast on his leg, which he detonated after he was stopped at a checkpoint about 65 feet meters from the mosque entrance. The other bomber made it inside as the prayers ended.
“Yemenis knew violence, but not this brutal,” said Farea al-Muslimi, a Sana-based political analyst, speaking about the assassinations, clashes and bombings over the last few days.
“There are no norms,” he said. “It’s a very scary moment.”