A group claiming to be a Yemeni branch of ISIS said it carried out the bombings and warned of an “upcoming flood” of attacks against the Houthi rebels, 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 the 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.

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, Al-Qaeda in the Arabian Peninsula. AQAP militants seized control Friday of a southern provincial capital, Al-Houta, in the most dramatic grab of territory by the group in years.

Meanwhile, the Houthi 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 Rabbou Mansour Hadi, has fled to the southern port city of Aden, where it battled Thursday with supporters of the former president, Ali Abdullah Saleh, who has allied with the Houthis.

AQAP has 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. The ISIS leader, Abu Bakr al-Baghdadi, formally accepted their oaths and declared a “province” of ISIS in Yemen in November. Baghdadi 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 are also frequented by Sunni worshippers. Footage from the Al-Hashoush 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. Nashwan al-Atab, a member of the Health Ministry’s operations committee, told AFP 142 people were killed and at least 351 wounded.

A prominent Shiite Imam, Al-Murtada al-Mansouri, and two senior Houthi leaders were among the dead, the rebel-owned Al-Masirah TV 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, Ahmad 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, 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-Hashoush 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 ISIS … 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, AQAP took control of the southern city of Al-Houta 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 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.