Yemeni forces have clashed with Islamist militants in the southern city of Zinjibar, leaving 15 people dead more than a week after hundreds of militants seized control of the city.

The fighting Tuesday comes amid increased calls for Yemen's president, Ali Abdullah Saleh, to accept a peace deal put forward by the Gulf Cooperation Council that would end his 33 years in power.

U.S. Secretary of State Hillary Clinton told reporters in Washington Monday that an “immediate transition is in the best interest of the Yemeni people.” Clinton noted that Mr. Saleh is currently out of the country, receiving medical treatment in Saudi Arabia for wounds suffered during Friday's rocket attack on his compound.

The Saudi government is also urging acceptance of the Gulf peace accord. And the European Union's foreign policy chief, Catherine Ashton, said Mr. Saleh should act “in the best interests of his people” and enable the country to “move on.”

Ashton noted that the Yemeni president had offered to sign the accord three times in recent weeks.

Yemen's acting leader, Vice President Abd al-Rab Mansur Hadi, was quoted as saying Mr Saleh is “making a strong recovery” and will return to the country within days. But there have been suggestions the Saudi government may block his return.

Clashes had diminished in Mr. Saleh's absence, although opposition tribesmen say government snipers in the Yemeni capital, Sana'a, have killed at least three supporters of tribal leader Sheikh Sadiq al-Ahmar.

Monday's shootings came a day after the vice president ordered government forces to vacate opposition-controlled areas of the capital, while al-Ahmar began to evacuate his militia from government buildings and called for an end to the fighting.

Nearly 400 people have been killed since the popular uprising against Mr. Saleh began in January.