AL MUKALLA, Yemen — The Islamic State extremist group claimed responsibility for a series of bombings in Yemen’s two largest cities on Tuesday that killed at least 25 people, including troops from the Persian Gulf who are fighting Yemeni rebels.
The first attacks — coordinated bombings in the southern city of Aden — were the largest in that city by the Islamic State since the group surfaced in Yemen last year.
Along with the bombing of a mosque in Sana, the capital, that killed at least seven people later on Tuesday, the attacks are the latest sign that Sunni extremists are gaining strength amid the country’s multisided war.
The bombings struck a hotel housing members of Yemen’s government, as well as a building that served as a headquarters for soldiers who belong to an Arab coalition that has been fighting Yemen’s Houthi rebels, according to security officials in Aden and a statement released by the Islamic State.
The group’s statement included photographs it said were of the bombers, some of them smiling, and called the attacks “a blessed operation.”
No government officials were wounded in the attack on the hotel. The official Emirati News Agency said four soldiers had been killed in Aden on Tuesday, without giving more details. A security official in Aden said at least 18 people had been killed in the attack on the soldiers’ headquarters. The official, who requested anonymity to speak about continuing security investigations, said it was not clear whether the victims were foreign soldiers or Yemenis allied with the Arab coalition.
The Islamic State has developed a reputation as Yemen’s most fearsome militant group, carrying out attacks that have killed dozens of people at a time — including some, on mosques, that are considered too extreme even by the local branch of Al Qaeda. Just as Al Qaeda has seized on the war to take control of territory, the Islamic State appears to be exploiting security lapses to carry out its own attacks while the Houthis and coalition forces are focused on fighting each other.
The bombings also demonstrated the security challenges for the government as it seeks to re-establish itself in Yemen after months of exile in Saudi Arabia. The government of President Abdu Rabbu Mansour Hadi was forced from power this year by the Houthis, who control Sana, the capital.
An Arab military coalition led by Saudi Arabia recaptured Aden from the Houthis in July.
The coalition, which includes the United Arab Emirates and eight other Arab states, has tried to consolidate its control over Aden and surrounding provinces and to rebuild the city after months of brutal fighting that destroyed neighborhoods and critical infrastructure.
Human rights groups and providers of emergency relief in Yemen have been deeply critical of the Saudi-led coalition, accusing it of indiscriminate attacks that have killed many civilians, including children.
Amnesty International, in a report issued Tuesday evening, said it had compiled “damning evidence of war crimes” by the coalition, based on an examination of 13 deadly airstrikes in northern Yemen that killed around 100 civilians.
Amnesty urged the United States and other weapons providers to halt arms transfers to the Saudis and their partners. The group also accused the coalition of using internationally banned cluster munitions in the conflict, a charge that Human Rights Watch also has made.
“More civilians have died as a result of coalition airstrikes than from any other cause during the conflict in Yemen,” Amnesty said.
Even if the Saudi-led coalition helps President Hadi reassert authority, there are fears that the south will face growing turbulence as a group of fighters who rallied behind the Saudis shift their focus from the Houthis to their own goals. That group includes fighters advocating a separate state in the south, as well as hard-line Sunni Islamists.
After the bombings on Tuesday, Yemeni officials tried to play down the possibility that they were carried out by Sunni extremists, quickly blaming the Houthis for what they said were a series of rocket attacks.
But security officials in the city said the attack on the hotel involved two car bombs, including one that detonated at the gate, and another that exploded inside the hotel garden. The government’s most senior official, Khaled Bahah, who serves as both prime minister and vice president, was evacuated from the hotel after the bombing, officials said.
In the other Aden attack, gunmen in armored vehicles stormed the Arab coalition’s headquarters, which a security official said was used mostly by Emirati soldiers. One of the armored vehicles, laden with explosives — the Islamic State said it was a Hummer — was detonated at the building, the security official said.