By AHMED AL-HAJ, Associated Press

SANAA, Yemen (AP) — An Islamic State-claimed suicide bomber and a subsequent car bombing killed at least 20 people Wednesday at a mosque in Yemen's rebel-held capital, Sanaa, amid the country's raging civil war, officials said.

The suicide bomber blew himself up inside the mosque during the evening call to prayers, while the car bomb exploded outside an entrance, they said. Medical officials said the death toll may rise with people now in operating rooms in several hospitals.

Witnesses said the car bomb exploded while people were carrying out the wounded from inside the mosque, adding to the casualties.

One witness, Hamid Ali, said the explosions left body parts and bloodied floors in the mosque frequented by both Sunni and Shiite Muslims. Those wounded pleaded for help.

In a message circulated on social media, Yemen's local Islamic State affiliate claimed the bombing, identifying the suicide attacker as Quay al-Sanaani and saying the assault was revenge against the Shiite rebels known as Houthis that hold Sanaa. The Associated Press could not independently verify the message, though it resembled other claims of responsibility by the group and was shared by extremist sympathizers.

The Islamic State affiliate in Yemen has carried out similar attacks targeting mosques, including a series of suicide bombings on March 20 in Sanaa that killed 137 people and wounding 345.

Yemen has been mired in violence since the Houthis captured Sanaa last September.

The Houthis are fighting alongside army units loyal to former President Ali Abdullah Saleh against forces loyal to exiled President Abed Rabbo Mansour Hadi as well as southern separatists and local militias. A Saudi-led and U.S.-backed coalition has been launching airstrikes against the rebels since March.

The conflict has killed over 2,100 civilians, according to the United Nations.

Earlier Wednesday, gunmen shot dead two Yemenis working for the International Committee of the Red Cross on Wednesday as they were traveling from the northern Saada province to the capital, Sanaa, the group said.

Rima Kamal, an ICRC spokeswoman in Sanaa, says the two were killed in Amran province.

Both Amran and Saada are fully controlled by the Houthis.

The U.N.'s humanitarian coordinator for the country, Johannes Van Der Klaauw, and U.N. humanitarian coordinator Stephen O'Brien both condemned the attack on the Red Cross workers.

Saudi Arabia's civil defense said Tuesday that seven people were wounded when a missile fired from inside Yemen struck three vehicles in al-Tuwal village in the Jizan border province.

Last month, pro-government forces backed by Saudi-led airstrikes drove the rebels out of Yemen's southern port city of Aden after heavy fighting.

In Marib province, more than 20 Houthis were killed in ground clashes with pro-government forces and in airstrikes by the Saudi-led coalition since Tuesday night, independent security officials and medical officials said. Nine pro-government fighters were also killed in the clashes in the same period, independent security officials and witnesses said.

Pro-government forces, who control the Marib province capital, are preparing for a large attack in the next two days, along with support from the Saudi-led coalition, anti-Houthi officials said. If they successfully clear the province of Houthi forces, the pro-government forces could then proceed to Jawf province, and then to Saada, the Houthis' stronghold in the north.

The officials spoke on condition of anonymity because they weren't authorized to brief reporters.

In a Wednesday report, Human Rights Watch says both sides committed serious abuses against civilians and fighters in their custody during fighting there, with southern militants killing at least seven Houthi prisoners since March.

"Southern forces that have regained control of Aden should end abuses against prisoners and do all they can to establish law and order in the city," said Sarah Leah Whitson, Middle East director for the U.S.-based rights group. "The Houthis need to release anyone wrongfully detained and account for everyone they are holding."