The US ambassador in Sanaa insisted Tuesday that Marine reinforcements deployed to Yemen were on a temporary mission with limited duties, a day after protests demanding their withdrawal and his expulsion.

"A small number of additional security personnel will temporarily assist with security and cleanup efforts at the US embassy in Sanaa," Ambassador Gerald Feierstein said in a statement on the embassy website.

"Their mission is strictly limited to assisting at our diplomatic facilities and protecting US diplomatic personnel from violence," he said in the statement posted a day after hundreds of students took to the streets to demand he be declared non grata.

"The group is assigned on a short-term basis in close consultation with the host government and operates in accordance with international law."

The Pentagon confirmed last Friday that the United States had deployed a 50-strong Marine counter-terrorism unit to Yemen to help protect the embassy in the face of protests by Muslims angered by a US-produced film that mocks Islam.

It followed a similar deployment to Libya after a assault on the US consulate in second city Benghazi killed Ambassador Chris Stevens and three other US officials.

Washington sought to make a third such deployment to Sudan after disturbances at the embassy there but was rebuffed by the Khartoum government.

The Yemeni parliament has criticised the deployment of the additional Marines but the Sanaa government insists it is a temporary measure to which it agreed.

On Thursday, an angry mob of protesters tried to storm the embassy grounds and clashed with Yemeni security forces, leaving four dead. The embassy has since suspended all consular services for two weeks.