Sana’a, Reuters—Suicide bombers in the Yemeni capital Sana’a blew themselves up during noon prayers at two mosques used mainly by Shi’ite Muslims on Friday, killing at least 50 people and wounding scores, medical sources said, in a major escalation of the worst wave of violence in the country in years.

The attacks, in which four bombers wearing explosive belts targeted worshipers in and outside the crowded mosques, happened a day after an unidentified warplane attacked the presidential palace in the southern city of Aden.

Anti-aircraft guns opened fire on planes flying high over the presidential compound in Aden on Friday, government sources and witnesses said.

There was no immediate claim of responsibility for the mosque bombings but supporters of Islamic State of Iraq and Syria (ISIS), an Al-Qaeda offshoot that has seized large areas of Iraq and Syria, used Twitter to welcome the attacks.

Yemen is torn by a power struggle between the Iranian-backed Houthi rebels in the north and the UN recognized President Abd Rabbuh Mansur Hadi, who has set up a rival seat in the south with the backing of Sunni-led Gulf Arab states.

The mosques in Sana’a are known to be used mainly by supporters of the Shi’ite Muslim Houthi group, which controls most of northern Yemen, including Sana’a.

The rise to power of the Houthis since September last year has deepened divisions in Yemen’s complex web of political and religious allegiances, and left the country increasingly cut off from the outside world.

One witness said he heard two successive blasts at one of the mosques, known as the Badr mosque, in a busy neighborhood in central Sana’a.

“I was going to pray at the mosque then I heard the first explosion, and a second later I heard another one,” the witness told Reuters.

Hospitals in Sana’a appealed for blood donors to help treat the large number of casualties. A Reuters witness at the scene of the Badr mosque said he counted at least 25 bloody bodies or corpses lying in the street and inside the mosque building.

One man carried a child in his arms.

The Houthi-linked Al-Masirah television channel showed young men dressed in traditional Yemeni clothes carrying lifeless bodies, some still dripping with blood, out of the mosque.

A third suicide bomber tried to blow himself up in one of the main mosques in the northern Houthi stronghold of Saada province but the bomb went off prematurely, killing the bomber only, a security source told Reuters.

Tensions have risen since Hadi fled to Aden in February after escaping a month of house arrest in Sana’a by Houthi forces in control of the capital.

Hadi has been trying to consolidate his hold over Aden, the better to mount a challenge to the Houthis’ ambitions to control the whole country.

Thirteen people were killed on Thursday when forces loyal to Hadi fought their way into Aden’s international airport and wrested an adjacent military base from a renegade officer, Aden governor Abdulaziz bin Habtoor said.

Both the fighting on the ground and subsequent air attack appeared to be part of a deepening power struggle between Hadi and the Houthi group, which is allied with former president Ali Abdullah Saleh, a fierce critic of Hadi.

On Friday, a cautious calm returned to Aden as the Yemeni president appeared to be consolidating his control over the city. The airport reopened and flights resumed as normal, an airport official told Reuters.