At least 137 people were killed and 357 wounded when suicide bombers, pretending to be disabled and hiding explosives under casts, attacked the mosques in Sanaa, according to Yemen's state-run Saba news agency.
Video distributed by Reuters showed people removing bodies from one of the mosques, where a carpeted floor was littered with debris.
If ISIS committed the attack, it would be not only a new challenge to the minority Houthis who took control of Sanaa weeks ago, but also a challenge to ISIS's rival, al Qaeda in the Arabian Peninsula, the Sunni extremists more associated with Yemen.
Regardless, Friday's attacks marked one of the worst days of recent violence during a complicated struggle for control of Yemen, where Houthi rebels -- Shiites in a predominantly Sunni country -- already faced resistance from AQAP and from supporters of ousted President Abdu Rabu Mansour Hadi.
A written statement, purportedly from ISIS, claimed that ISIS executed Friday's attacks, calling them "a tip of an iceberg."
The statement, posted on a site that previously carried ISIS proclamations, said five suicide bombers targeted Houthis in Sanaa.
A separate audio message, also posted on ISIS-affiliated websites, claimed five ISIS suicide bombers killed dozens of "Houthi infidels." The voice is similar to one featured in Thursday's audio message in which ISIS claimed responsibility for Wednesday's attack at the Bardo Museum in Tunisia
CNN cannot independently verify the legitimacy of Friday's statements.
Among those killed in Sanaa was prominent Houthi religious leader Murtatha Al Mahathwari, Saba reported.
A separate explosion rocked a government compound in the Houthi stronghold city of Saada -- 180 kilometers (112 miles) northeast of Sanaa -- killing two people and seriously wounding a third, according to Abu Khalil Al Ameri, a local Houthi security official.
If ISIS executed Friday's bombings, it would be a "big deal indeed," in part because ISIS was thought to have only a fledgling presence in Yemen, CNN terrorism analyst Paul Cruickshank said.
"The dominant group there is al Qaeda in Yemen. ISIS and al Qaeda in Yemen can't stand each other," Cruickshank said. "But ... whoever is responsible, I think, (is) trying plunge the country into civil war."
That's a complication for a number of parties, including the United States, which until the Houthis took Sanaa had counted on the Yemeni government for support in its long-running battle against AQAP.
Blasts inside, outside mosques
In Friday's assaults at Al Badr mosque and Al Hashoosh mosque in Sanaa, the first blasts happened inside the buildings, followed two minutes later by explosions outside, perhaps to target those fleeing the preliminary blasts, two senior Houthi leaders in Sanaa said.
At Al Badr mosque, the outdoor explosion was another suicide bombing; at Al Hashoosh mosque, the exterior blast was a car bomb, the two leaders said.
"We check people and watch at times, but it's a mosque, and we can't check everyone who enters," said Ali Al Emad, a Houthi security worker at Al Hashoosh mosque.
The mosques serve members of the Zaidi sect of Shiite Islam, the sect to which the rebel Houthi militants belong.
Among the wounded are 40 people who are in critical condition; they will be sent to Jordan for treatment, reported Saba, which is controlled by the Houthis.
AQAP, which last year vowed to attack Houthi loyalists throughout Yemen, issued a statement saying it had nothing to do with Friday's bombings.
Houthis have Sanaa, launched airstrike in Aden
The mosque attacks came six months after the Houthis -- who have long felt marginalized by the majority Sunnis in Yemen
and have battled the central government for more than a decade -- entered Sanaa. That sparked battles that left than 300 people dead before a ceasefire was agreed to that month.
The Houthis gradually took control, seizing the presidential palace in January and forcing President Hadi to resign.
Hadi initially was put under house arrest, but he escaped last month, fleeing to the southern port city of Aden and declaring himself still President.
The Houthis took control of military forces stationed near Sanaa, including the air force, as they overtook the central government there in January.
Though the Houthis hold sway in Yemen's north, they have less influence in the south. Clashes between Houthi-controlled forces and military units still loyal to Hadi stepped up this week
in Aden, about 300 kilometers (186 miles) southeast of Sanaa.
Thursday, a Yemeni jet commanded by the Houthis fired missiles at a palace where Hadi was taking refuge in Aden, injuring no one but marking an escalation in the fighting.
The jet flew from Sanaa to the palace in Aden, where the jet conducted the strikes, a senior air force official said on condition of anonymity.
Hadi was at the palace compound when the first missile struck the grounds, but he fled safely, a Hadi aide said, also on condition of anonymity.
A second missile struck near the compound but, like the first, injured no one, two officials in Aden said.
The airstrikes came on the same day that opposing Yemeni military forces -- those loyal to the Houthis, and those led by officers loyal to Hadi -- battled in Aden, leaving at least 13 people dead and 21 others injured, Aden Gov. AbdulAziz Hobtour said.
Some of the forces loyal to the Houthis also are loyal to Hadi's predecessor, former President Ali Abdullah Saleh, who resigned in 2012 after months of "Arab Spring" protests inspired in part by a 2011 revolution in Egypt. Hadi was Saleh's vice president before taking over.
Houthi takeover stunned West
The United States, which still considers Hadi to be Yemen's legitimate President, "strongly condemns" both Friday's mosque bombings and Thursday's airstrikes in Yemen and expressed its condolences to the victims' families and loved ones, according to a White House statement.
"This unconscionable attack on Muslim worshipers during Friday prayers only further highlights the depth of the terrorists' depravity and the threat they pose to the people of Yemen, the region, and the world," read the statement. "Today's attacks underscore that terrorism affects all Yemenis and that Yemen and its people must remain unified to confront these challenges."
An earlier statement from U.S. State Department spokesman Jeff Rathke specifically called on "the Houthis, former President Ali Abdullah Saleh, and their allies to stop their violent incitement and undermining of President Abdu Rabu Mansour Hadi ... the way forward for Yemen must be through a political solution."
Officials in Saudi Arabia urged all Yemeni political groups to take part in talks in Riyadh, the state-run Saudi Press Agency said.
King Salman ordered medical aid be sent to the victims in Sanaa and Aden, SPA reported, and said his country was ready to receive anyone critically wounded in the attacks.
The Houthi takeover of Sanaa stunned governments of Western nations, including the United States, which had a long relationship with Yemen's leader and worked with the regime to target AQAP militants.
U.S. officials frequently say AQAP is one of the most dangerous terrorist groups in the world, based on its attempts to attack U.S. interests, including an attempt to blow up an American jetliner over Detroit in December 2009.
The United States, along with most European and Persian Gulf countries, suspended operations in their embassies this year after the Houthis took Sanaa.
But the United States' anti-AQAP drone program in Yemen continued, with a U.S. drone strike killing senior AQAP cleric Harith bin Ghazi al-Nadhari and three other people in Shabwa province on January 31.