ADEN, Yemen -- Quadruple suicide bombers on Friday hit a pair of mosques controlled by Shiite rebels in the Yemeni capital, Sanaa, unleashing blasts through crowds of worshippers that killed at least 137 people and wounded around 350 others in the deadliest violence to hit the fragile war-torn nation in decades.
A group claiming to be a Yemeni branch of the Islamic State of Iraq and Syria, or ISIS, said it carried out the bombings and warned of an "upcoming flood" of attacks against the rebels, known as Houthis, who have taken over the capital and much of Yemen. The claim, posted online, could not immediately be independently confirmed and offered no proof of an ISIS role.
If true, Friday's bombing would be the first major attack by ISIS supporters in Yemen and an ominous sign that the influence of the group that holds much of Iraq and Syria has spread to this chaotic nation. The claim was posted on the same web bulletin board where the ISIS affiliate in Libya claimed responsibility for Wednesday's deadly attack on a museum in Tunisia.
"The tragedy in Yemen today underscores that the terrorist risks are real, that you have violent extremist groups that continue to attack in Yemen and no doubt are continuing to try to plot an attack in the West," CBS News senior national security analyst Juan Zarate told CBS Radio News.
A significant presence of ISIS supporters would add an alarming new element to the turmoil in this fragmenting nation. Yemen is already home to the most powerful branch of the al Qaeda network - which is a rival of ISIS. On Friday, al Qaeda militants seized control of a southern provincial capital, al-Houta, in the group's most dramatic grab of territory by the group in years.
Meanwhile, the Shiite rebels' capture of the capital and a large swath of the country - at least nine of its 21 provinces - has raised fears of a civil war tinged with sectarianism. The government of the internationally backed president, Abed Rabbo Mansour Hadi, has fled to the southern port city of Aden, where it battled on Thursday with supporters of the former president, Ali Abdullah Saleh, who has allied with the Houthis.
Al Qaeda in the Arabian Peninsula, as the Yemeni branch is called, has also been battling for months against the Houthis in various parts of the country. But the group issued an official statement denying it carried out Friday's bombings, pointing to earlier instructions from the terror network's leader Ayman al-Zawahri not to strike mosques or markets.
In Washington, White House spokesman Josh Earnest said the U.S. had seen no indications of an operational link between ISIS and Friday's attacks. He said the U.S. was investigating to see whether the ISIS branch in Yemen has the command-and-control structure in place to substantiate its claim of responsibility.
Earnest said it was plausible that ISIS was falsely claiming responsibility for the incident. "It does appear that these kinds of claims are often made for a perception that it benefits their propaganda efforts," Earnest said.
In past months, there have been several online statements by individual Yemeni militants declaring allegiance to ISIS. Its leader, Abu Bakr al-Baghdadi, formally accepted their oaths and declared a "province" of ISIS in Yemen in November. He and his deputies in Iraq have vowed to strike against the Houthis in Yemen. That has raised questions whether direct operational links have also arisen. For example, In Libya, where ISIS also declared official "provinces," fighters and officials are known to have been sent from the group's core to build the local branch.
Friday's bombings left scenes of bloody devastation in the Badr and al-Hashoush mosques, located across town from each other in Sanaa. Both mosques are controlled by the Shiite Houthis, but they also are frequented by Sunni worshippers. Footage from the al-Hashoosh mosque showed screaming volunteers using bloodied blankets to carry away victims, with a small child among the dead lined up on the mosque floor.
Two bombers hit each mosque during midday Friday prayers, when large crowds turn out to attend weekly sermons. The rebel-owned Al-Masirah TV channel said the casualty figures had reached 137 dead and 345 injured. A prominent Shiite cleric, al-Murtada al-Mansouri, and two senior Houthi leaders were among the dead, the TV channel reported.
It also reported that a fifth suicide bomb attack on another mosque was foiled in the northern city of Saada - a Houthi stronghold.
In the Badr mosque, the first bomber was caught by guards searching worshippers at the gate, where he detonated his device. In the ensuing panic, a second bomber entered the mosque and blew himself up amid the crowd, according to the official news agency SABA.
"I fell on the ground and when I regained consciousness I found myself lying in a lake of blood," one survivor, Ahmed al-Gabri, told the AP. Two worshippers next to him were killed in the explosions, then another died when one of the mosque's large glass chandeliers fell on him, al-Gabri said.
Another survivor, Sadek al-Harithi, said the explosions were like "an earthquake where I felt the ground split and swallow everyone."
In the al-Hashoosh mosque, one witness said he was thrown two meters away by one of the blasts and found the floor strewn with body parts.
"Blood was running like a river," Mohammed al-Ansi said.
In an online statement, a group calling itself the media office of ISIS' "Sanaa Province" claimed responsibility for the attack, saying that the four Sanaa suicide bombers blew themselves up among crowds of Houthis.
"This operation is just a glimpse of an upcoming flood, God willing," the group said in the statement. "We swear to avenge the blood of Muslims and the toppling of houses of God."
"The soldiers of the Islamic State ... will not rest until we have uprooted (the Houthis), repelled their aggression, and cut off the arm of the Iranian project in Yemen," it said, referring to claims that Shiite powerhouse Iran is backing the rebels.
In a further sign of the country's chaos, al Qaeda's branch of the country took control of the southern city of al-Houta on Friday, Yemeni security officials said. Al Qaeda militants driving pickup trucks and flying black flags swept through the city, which is the capital of Lahj province. They took over the main security barracks, the governor's office, and the intelligence headquarters, which houses prisons with al Qaeda detainees, the officials said.
Most of the security forces in the city - loyalists of the former president, Saleh - surrendered to the militants without resistance. The militants killed 21 members of the security forces who resisted at the governor's office, the officials said, speaking on condition of anonymity because they were not authorized to speak to the press.