CAIRO: Yemeni rebels ambushed pro-government forces in the south Tuesday, setting off a major battle that killed 65 anti-rebel forces and handed them their first serious setback following a series of recent advances, officials said.

The officials said 15 Houthi rebels were killed in the fighting near the Aqaba Tharaa area, where anti-rebel forces were advancing from Abyan into Bayda province.

The officials, who hailed from both sides of the conflict, said the rebels destroyed at least eight armored vehicles and four tanks, which were left burning. The officials said both sides were rushing reinforcements to the area, with the Houthis attempting to squeeze the pocket closed and their opponents attempting to break out.

Dozens of airstrikes by the Saudi-led coalition opposing the rebels hit the mountainous area, but had yet to open up the rebels’ positions.

All officials spoke on condition of anonymity because they were not authorized to brief the media.

The Saudi-backed groups fighting the Houthis and their allies have made rapid gains in the south in recent weeks, after taking the southern port city of Aden in July. Using Aden as a resupply point, they have moved forces, including Saudi-trained Yemenis and armored units, northward, seizing Shabwa province from the rebels last weekend.

In the west, the officials and witness said Saudi-led airstrikes hit several warehouses in the coastal city of Hudaida, leaving them in flames after weapons stores went off.

The Houthis have meanwhile taken control of the United Arab Emirates embassy in the capital Sanaa, prompting condemnation from Emirati and Egyptian authorities. Egypt’s Foreign Ministry Tuesday urged the Houthis to withdraw from the premises following the seizure Sunday. The Emirates, along with Saudi Arabia, Bahrain and Qatar, have also condemned the takeover of the embassy. All are part of the Saudi-led coalition that has been bombing the Houthi rebels and their allies since March.

Earlier Tuesday, a leading international rights group said that all sides fighting in Yemen have left a “trail of civilian death and destruction” in the conflict, killing scores of innocent people in what could amount to war crimes.

Amnesty International said the violence has been particularly deadly in the southern cities of Taiz and Aden, with dozens of children among those killed.

“Civilians in southern Yemen have found themselves trapped in a deadly crossfire between Houthi loyalists and anti-Houthi groups on the ground, while facing the persistent threat of coalition airstrikes from the sky. All the parties to this conflict have displayed a ruthless and wanton disregard for the safety of civilians,” said Donatella Rovera, Amnesty’s senior crisis response adviser.

“The report depicts in harrowing detail the gruesome and bloody trail of death and destruction in Taiz and Aden from unlawful attacks, which may amount to war crimes, by all parties,” Amnesty said.

Amnesty, Human Rights Watch and other rights groups have repeatedly expressed concern that both the Saudi-led coalition and Houthi-allied forces were violating the laws of war and not doing enough to prevent or minimize civilian casualties.

Amnesty has previously said that evidence suggested the Houthis carried out indiscriminate mortar attacks on civilians and repeatedly targeted medical workers and facilities in Aden.

The coalition has denied targeting civilians. “I don’t think they have a very accurate report. They never contacted us to ask any clarification for any situation,” Saudi-led coalition spokesman Brig. Gen. Ahmed Asseri said, adding that some of the strikes Amnesty attributed to the coalition were in fact Houthi missiles.

In Tuesday’s report, Amnesty catalogued a series of incidents involving both air and ground operations. During its June-July research mission to Yemen, Amnesty investigated eight airstrikes by the Saudi-led coalition, which hit heavily populated areas mostly with no nearby military targets, killing at least 141 civilians and wounding 101 others, mostly women and children.

The group said it also investigated dozens of incidents of ground combat, where both sides routinely used weapons such as Grad-type rockets, mortars and artillery shells in densely populated residential areas. In Aden and Taiz, it said at least 68 civilians were killed and 99 wounded in such attacks.

One of the deadliest attacks was on July 19, when the Houthis and their allies shelled the Dar Saad neighborhood of Aden, killing 45 people, mostly civilians, Amnesty said.

Earlier this month, the U.N. Office of the High Commissioner for Human Rights said that at least 1,916 civilians have died in the Yemen conflict since it escalated on March 26.