Yemeni authorities declared a state of emergency in a southern provincial capital today, citing the possibility of separatist attacks two days after a policeman was shot dead in an ambush in a nearby province.

Yemen said separatists killed the officer in south Yemen on Thursday, bringing to four the number of people killed in attacks on southern security men in a week as authorities also mounted sweeps targeting separatists.

A government official said the state of emergency in the city of Dalea was "to guard against acts of violence that outlaws are intent on carrying out in the city". He spoke in language Sanaa typically uses to refer to separatists.

"(The move) is aimed at securing the lives of citizens and maintaining provincial security and stability after reports of the entry of armed elements to the municipality to carry out conspiratorial plots, disturb the peace and harm the public interest," the official told Reuters, declining to be named.

Yemen, in addition to its conflict with separatists, is also facing a northern Shi'ite insurgency and a resurgent al Qaeda, whose Yemen-based regional arm claimed responsibility for a failed December bomb attack on a US-bound plane.

Western governments and neighbouring Saudi Arabia, the world's biggest oil exporter, fear al Qaeda is exploiting instability on several fronts in Yemen to recruit and train militants to launch attacks in the region and beyond.

People in south Yemen, home to most Yemeni oil facilities, complain that northerners have abused a 1990 agreement uniting the country to grab resources and discriminate against them.

Tension has flared in recent weeks in the south after a separatist protester was killed on 13 February when police opened fire at a demonstration. Six others were injured.

Police later clashed with demonstrators who came to claim the protester's body, igniting a week of unrest in which separatists burned northern-owned shops and tried to block a road linking Lahj province to the main southern city of Aden.

Security officials have since launched sweeps in which at least 130 people were arrested across four southern provinces including Dalea. Government officials said a curfew would be imposed in Dalea from dusk as part of measures imposed under the state of emergency.

"The decision includes the closure of all main and side entrances to the city, prevents the entry of outlaws and bars demonstrations or unlicensed gatherings," one official said.

Four people have been killed in attacks on security forces in roughly a week. Separatists killed two people including a senior police official eight days ago in an ambush, and on Monday a military officer was killed in south Yemen.

On Thursday in Zanjibar, witnesses said they heard heavy fire from gunmen thought to belong to a separatist militia, and later saw a 45-year-old officer slumped dead on his motorcycle.