Four Islamists linked to al Qaeda and a soldier have been killed in clashes in southern Yemen, security sources said Sunday, hours after the government said it had begun a cease-fire to secure the area before a presidential election later this month.

The clashes between militants from Ansar al-Sharia (Partisans of Islamic Law) and troops broke out late Saturday on the outskirts of Zinjibar, capital of Abyan province, which militants seized in May.

The capture of the coastal city has increased the vulnerability of nearby Aden, Yemen's second city and an important port, security experts say. The United States and Saudi Arabia are particularly concerned that emboldened militants linked to al Qaeda could threaten nearby oil shipping routes through the Red Sea.

Diplomats hope the presidential elections, agreed as part of a deal to ease President Ali Abdullah Saleh out of office, will pull the country back from the brink of civil war.

Zinjibar has been nearly destroyed in clashes between the government and Islamist militants in the past eight months. Tens of thousands have fled and are now refugees in Aden and Lahej.

Tribesmen and residents said government mediators on Saturday forged a cease-fire with militants in an effort to create a peaceful environment for voting on February 21, but militants denied they had made any such agreement.

They (the government) wanted to hold negotiations with Ansar al-Sharia, but (we) refused, a representative of the Islamist group told Reuters.

A government official said authorities opted to let tribesmen negotiate with militants in Zinjibar rather than fight, partly because of divisions within the army between Saleh loyalists and those who oppose the exiled leader.

VIOLENCE AHEAD OF ELECTIONS

In the southeastern city of Mukalla one protester was killed and six wounded when security forces attacked a sit-in by southern separatists calling for an election boycott, an activist told Reuters. There was no immediate comment by officials.

North and south Yemen were only united in 1990 and after Saleh's forces crushed a southern successionist campaign four years later. The south is home to much of Yemen's oil and many living there say northerners have seized their resources and discriminate against them, a charge Sanaa denies.

The election is also opposed by Islamist militants and northern Shi'ite Muslims rebels known as Houthis, who have been left out of the political deal which has transferred power to an opposition led-government.

Militants exploited last year's violent political struggle against Saleh's 33-year rule to seize towns such as Zinjibar and Radda in Yemen's mountainous interior.

Militants quit Radda in exchange for the release of jailed comrades and the formation of a council to govern the town.

However, a similar solution in Abyan province is not likely given that militants there are firmly entrenched.

Highlighting the fragile security situation just weeks before voting, militants Saturday attacked a Yemeni army base in the south. In another part of Abyan, unknown gunmen opened fire on a group of tribesmen, killing one and wounding two. Some tribes have joined forces with the army against the militants.

(Reporting by Mohammed Mukhashaf; Writing by Nour Merza; Editing by Sami Aboudi and Ben Harding)