Shiite rebels and their allies in Yemen randomly shelled a town Sunday outside of Aden after losing control of some the port city's neighborhoods, killing at least 100 people and wounding 200, the head of an international aid group said.

Hassan Boucenine of the Geneva-based Medicins Sans Frontieres (MSF), or Doctors Without Borders, gave the new toll on Monday. He told The Associated Press that the victims in the town of Dar Saad are mostly civilians and that MSF fears "attacks on civilians will continue."

The violence highlighted the bloody chaos of the civil war gripping the Arab world's poorest country, which also has been the target of Saudi-led, U.S.-backed airstrikes since late March.

A leader with the Shiite rebels, known as Houthis, denied shelling Dar Saad, which is just north of Aden and long home to fighters resisting their advances. But Boucenine and Yemeni medical officials said the shelling clearly came from the north and east of Dar Saad — areas under rebel control.

Aden, the scene of some of the war's fiercest ground battles, saw Saudi-backed troops and fighters seize from the Houthis some of its neighborhoods and its international airport last week. Sunday's shelling in Dar Saad appeared to be a way to both punish those resisting the Houthis, as well as halt the advance of their opponents.

Abdu Mohammed Madrabi, 65, said he was in line outside the post office to collect his pension when the shells hit, causing chaos. Madrabi, who was wounded in the neck, back and leg, said many private cars carried the wounded to hospitals because there weren't enough ambulances.

The shelling was intense in the neighborhood of Sharqiya, hundreds of yards from the post office.

"It's been one shell after the other since the morning," said Arwa Mohammed, a resident of Sharqiya locked up in one room with her seven-member family for safety. "We are feeling the house is going to collapse over our head."

Anis Othman, a neighbor of Mohammed, also described a scene of pandemonium.

"Balls of fire are falling over our heads amid the screams of children and women," he said. "Why all that shelling? There are no weapons or fighters here. They (the rebels) want to terrorize us and drive us out. This is only rancor and hate."

Zeifullah al-Shami, a Houthi leader, denied targeting civilians in the shelling, saying his forces were engaging the rivals on the front lines.

"This is part of the media deception," he said. "We didn't kill civilians."

However, the rebels had vowed to retaliate after losing ground in Aden. The rebels now are largely based in Aden's western neighborhood of Tawahi, as well as bases east of Aden and in Lahj province, north of the city. Saudi-backed fighters also are advancing on a military air base in Lahj province.

Sunday night, anti-Houthi forces linked up in Tawahi from the north and south at the state television building, a Yemeni military official said. He claimed anti-Houthi forces fully controlled the area and said they were searching for rebels, some of whom had fled to nearby mountains. He too spoke on condition of anonymity as he wasn't authorized to speak to journalists.

The fighting in Yemen pits the Houthis and troops loyal to former President Ali Abdullah Saleh against southern separatists, local and tribal militias, Sunni Islamic militants and loyalists of exiled President Abed Rabbo Mansour Hadi, who is now based in Saudi Arabia.

The rebels seized the capital, Sanaa, in September. Fierce fighting in Aden broke in March, sparking the Saudi-led airstrikes. More than 3,000 people have been killed since, including more than 1,400 civilians, according to United Nations agencies.

The conflict has left 20 million Yemenis without access to safe drinking water and uprooted more than 1 million people from their homes, the U.N. has said.