Yemen has urged the UN to authorise the use of foreign ground forces to drive back Houthi rebels, specifically in the cities of Aden and Taiz.

Rebels have been advancing in both areas despite air strikes by a Saudi-led coalition.

The strikes began after Yemen sent the UN a similar letter in March requesting military help from Gulf Arab states.

The latest request also urged human rights groups to document "barbaric violations" by Houthis.

The letter, from Yemen's UN ambassador Khaled Alyemany, cited an incident on Wednesday when at least 32 people were killed while trying to flee Aden in a boat.

The letter said Houthis were "targeting anything that moves" in Aden.

"We urge the international community to quickly intervene by land forces to save Yemen, especially Aden and Taiz," it said.

Civilians trapped

Fighting has been particularly fierce in Aden's al-Tawahi district where rebels have been battling pro-government forces for control.

The international community has expressed growing concern for civilians trapped in the fighting.

Yemenis walk on a blocked road leading to the headquarters of the Shiite Houthi rebel movement following recent clashes between Houthi fighters and Yemeni security forces on June 21, 2014 in the Yemeni capital Sanaa. AFP PHOTO / MOHAMMED HUWAIS.

More than 20 international aid agencies have warned fuel shortages in Yemen could halt their work.

The Saudi-led coalition aims to restore the government of exiled President Abdrabbuh Mansour Hadi but has so far failed to stop the Houthis' assault on Aden.

The president fled the capital Sanaa in February and took refuge in Aden. When the Houthis reached the outskirts of Aden at the end of March, he left the country for Saudi Arabia.

Since then, more than 640 civilians have been killed, according to the UN.

US Secretary of State John Kerry called for a pause in the fighting when he arrived in the Saudi capital Riyadh on Wednesday.