The Islamic State in Iraq and Syria (ISIS) is claiming responsibility for two bombings that killed at least 20 people at a mosque in Yemen's capital city on Wednesday.

A suicide bomber blew himself inside the mosque in Sanaa during the evening call to prayers and then a car bomb exploded outside the entrance, officials 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 Shia Muslims.  

In a message circulated on social media, Yemen's local ISIS affiliate said it was behind the bombing, identifying the suicide attacker as Quay al-Sanaani. The statement said the assault was revenge against the
Shia rebels known as Houthis who hold Sanaa amid Yemen's raging civil war. 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 ISIS 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 wounded 345.

Bombing follows Red Cross attack

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 as they were travelling from the northern Saada province to the capital, the aid group said.

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

Both Amran and Saada are fully controlled by the Houthis.

The UN's humanitarian co-ordinator for the country, Johannes Van Der Klaauw, and UN humanitarian co-ordinator Stephen O'Brien both condemned the attack on the Red Cross workers.

Saudi Arabia's civil defence 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.

Large attack planned

In Marib province, more than 20 Houthis have been 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 stronghold of the Houthis in the north.

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

In a Wednesday report, Human Rights Watch said both sides have 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."