An attack on Huthi rebel leaders in Yemen's capital claimed by the Islamic State group has killed at least 28 people, medics said Tuesday, in the latest anti-Shiite assault by the Sunni extremists.
Meanwhile, rebels fleeing pro-government forces who captured a jail from them threw open its doors and freed around 1,200 prisoners, a loyalist source said.
Yemen was previously the preserve of IS's jihadist rival Al-Qaeda, which controls swathes of the south and east, but IS has claimed a string of high-profile attacks since March.
A car bomb late Monday targeted two brothers, both rebel chiefs, during a gathering to mourn the death of a relative, a security source said.
Eight women were among the dead.
IS said online it had organised the attack on a "Shiite nest".
The group considers Shiites heretics and has repeatedly targeted them, not only in Yemen but across the region.
Just Friday, a Saudi IS suicide bomber killed 26 people and wounded 227 in a Shiite mosque in Kuwait.
In Yemen, IS claimed a car bombing that killed two people outside a Shiite mosque in Sanaa on June 20 and a series of attacks in the capital four days earlier that killed 31.
IS, which Monday marked the first anniversary of its declaration of a "caliphate" in Iraq and Syria, launched its Yemen campaign in March with a series of bombings of Shiite mosques that killed 142 people.
The attacks have overshadowed the operations of Al-Qaeda in the Arabian Peninsula, which overran Mukalla, capital of Hadramawt province in southeast Yemen, in March.
Washington still regards AQAP as the network's most dangerous branch and has kept up a drone war against its leaders inside Yemen.
But analysts say IS is now clearly in the ascendant.
IS is "in the process of supplanting AQAP, which is becoming just one of a number of forces in the Sunni tribal camp in southern Yemen," said Mathieu Guidere, Islamic studies professor at France's University of Toulouse.
The Iran-backed Huthi rebels have seized vast swathes of Yemen since launching an offensive last July, forcing President Abedrabbo Mansour Hadi to flee to Saudi Arabia.
- 'New surprises' -
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Neither peace talks in Geneva nor a Saudi-led air campaign begun in March have driven them from power, and they remain locked in battle with pro-government fighters, Sunni tribesmen and southern separatists.
On Tuesday, Huthi rebels fleeing an attack by pro-government forces released around 1,200 prisoners from a jail, a loyalist source said, in what may have been an attempt to cover their withdrawal.
The captives, including convicted murderers, were being held outside Taez in southwestern Yemen, when the jail was overrun by fighters of the pro-government's Popular Resistance Committees.
Another loyalist source said their forces were now trying to round up the escapees.
Yemen's official Saba news agency, controlled by the Huthis, reported early Tuesday that the rebels had fired a Scud missile at Al-Salil military base in Riyadh province, deep inside Saudi Arabia.
"This is another message to the forces of oppression," a military spokesman was quoted as saying, promising "new surprises in the coming days".
Saudi Arabia denied a Scud had hit its territory.
"We're not aware of anything. Not a square metre (yard) was damaged in Saudi Arabia," an official in Riyadh told AFP, adding there had been no radar contact and no missile intercepted.
A Saudi civil defence spokesman, however, said two civilians were lightly wounded Tuesday when a projectile fired from inside Yemen hit their house.
Bombardments and skirmishes along the border have killed at least 45 Saudi troops and civilians, including a soldier hit by rebel fire on Monday.
In the southern port of Aden, Yemen's second city, fighting raged Tuesday between the rebels and their opponents.
A pregnant woman and two children were among 13 people killed over the past 48 hours, medics said. Another 216 people were wounded.
Oil tanks at the city's refinery were still ablaze after being hit by rebel fire Saturday.
Saudi-led aircraft carried out 20 strikes in support of loyalists, a local official said.
Human Rights Watch said on Tuesday air strikes on the rebel stronghold of Saada in Yemen have destroyed houses, markets and a school, killing dozens of people in what could amount to war crimes.
And on Monday, UN chief Ban Ki-moon called for an investigation after Saudi-led air strikes hit the UN Development Programme compound in the city, wounding a guard and causing serious damage.
A local official accused the rebels of firing on a Qatari aid ship preventing it from docking in the city, which is in desperate need of relief supplies.
Ismail Ould Cheikh Ahmed, the UN's envoy to Yemen, made a new plea for a humanitarian pause when he met President Hadi in Riyadh Tuesday, a government source said.