SANAA, Yemen -- Yemen's Shiite rebels and their allies consolidated their hold in another part of the southern port city of Aden on Wednesday after heavy fighting with militiamen loyal to the exiled government and capturing the area's presidential palace, officials said.

The latest advance by the rebels, known as Houthis, and their allies, loyalists of former president Ali Abdullah Saleh, showed they are still combat ready despite Saudi-led airstrikes.

The rebels took Aden's southwestern Tawahi district, killing the area's military commander, Maj. Gen. Ali Nasser al-Hassani, and seizing the palace, military officials said.

U.S. forces once used the palace as an operations centre and training facility for anti-terrorism forces after rebels captured Sanaa last September, they said. Experts and trainers left this spring when clashes began in Aden.

Gunmen reportedly shot at residents trying to flee Tawahi by boat, while several hundred families managed to flee elsewhere in Aden by sea, said Farhan Haq, the deputy spokesman of the United Nations secretary-general.

Security officials said at least 48 civilians were killed when an artillery shell landed near a boat they were attempting to use to flee the neighbourhood, with some of them drowning after the boat capsized.

Meanwhile, the security officials said airstrikes continued throughout the country, killing dozens of rebels and their allies. They spoke on condition of anonymity as they weren't allowed to brief journalists.

The UN human rights office says at least 646 civilians have been killed in Yemen since the airstrikes began March 26. President Abed Rabbo Mansour Hadi's government in exile in Saudi Arabia puts the civilian death toll at least 1,000. At least 300,000 people have been displaced.

Yemeni Foreign Minister Riad Yassin urged the international community to intervene to rescue civilians he said the Houthis deliberately targeted.

"We appeal coalition forces to do all they can as soon as possible to salvage Aden," he said, adding that any negotiations over the country's future could not include Saleh or his family members.

U.S. Secretary of State John Kerry said he hopes to secure a pause in the war as he prepared for Thursday talks in Saudi Arabia.