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Four suicide bombers hit a pair of crowded mosques in Yemen's capital of Sanaa on Friday, killing at least 137 people and injuring more than 300 others, officials told NBC News.

The ISIS affiliate in war-torn Yemen claimed responsibility for the attacks, according to Flashpoint Intelligence, a global security firm and NBC News consultant. It was the first large-scale attack claimed by the Sunni militants in Yemen, which has been in a state of chaos since Shiite Houthi rebels launched a violent power grab.

Dr. Alia Saria, head of emergency services at Yemen's Ministry of Public Health and Population, confirmed the death toll to NBC News and said "hundreds" were injured. Mohammed Albasha, Yemen's spokesperson in Washington, put the number of injured above 300. Albasha said the bombers struck Badr and al-Hashoosh mosques during Friday prayers — traditionally the busiest time of the week. Both mosques were hit by two bombers using similar tactics: one would detonate explosives inside the building while the second waited outside for people to flee before blowing himself up, Albasha explained.

One witness at the al-Hashoosh mosque told the AP he was thrown two meters by the blast.

"The heads, legs and arms of the dead people were scattered on the floor of the mosque," Mohammed al-Ansi told the AP. "Blood is running like a river."

"I found myself sleeping in a lake of blood"

Another survivor from the Badr mosque, Ahmed al-Gabri, told AP: ""I fell on the ground and when I regained conscious I found myself sleeping on a lake of blood."

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Hospitals are doing "what they can" to treat an influx of wounded, according to Dr. Mohammed Musoke, a medical coordinator with Médecins Sans Frontières (MSF) in Sanaa.

"Most of the operating theaters are full but the hospitals are doing what they can," he said.

A large number of children were among the injured receiving treatment, according to the International Committee of the Red Cross (ICRC).

"There were a lot of children because there were a lot of boys going to pray with their fathers," said Marie Claire Feghali, the ICRC's spokeswoman in Sanaa. "There were many blast injuries and fractures."

Yemen's ISIS affiliate also said a fifth bomber had carried out a separate attack in the northern city of Sa'adah. However, Albasha told NBC News that a bomber there was stopped by security as he tried to enter a mosque, killing only himself and causing no other casualties.

Friday's attacks were the deadliest to hit Yemen in years, though the Arab world’s poorest country has been gripped by a spiraling crisis since January when Iran-backed Shiite Houthi rebels seized large parts of the capital — including President Abd-Rabbu Mansour Hadi's official residence.

The Houthis are allied with former president Ali Abdullah Saleh, who still wields influence in the armed forces despite having given up power in 2011. Their advance on Sanna prompted Yemen's U.S.-backed president, prime minister and cabinet to abruptly resign.

Amid the subsequent power struggle and deteriorating security, the U.S., U.K. and France closed their embassies and urged their citizens to leave the country.

Yemen’s crisis also reportedly “paralyzed” U.S. counter-terrorism operations against al Qaeda in the region. Many U.S. personnel work with Yemeni forces at the southern al-Annad airbase, a key intelligence post and drone strikes launch from there have killed dozens of suspected AQAP fighters and leaders.

The United Nations Security Council condemned the attack, and said in a statement that the terrorists who carried it out must be brought to justice. “Terrorism in all its forms and manifestations constitutes one of the most serious threats to international peace and security,” the statement said.

Clashes between the two sides erupted on Thursday in Yemen's city of Aden. At least 63 patients — most suffering from gunshot wounds — were treated in the Doctors Without Borders hospital in the city, the organization said in a statement.

Outside the capital, the Houthis have been fighting al-Qaeda-linked terror group Ansar al-Sharia, raising fears that Yemen could descend into a Libya-style chaotic fight between rival tribal groups and between those groups and Islamist militants.

— The Associated Press and Reuters contributed to this report.