A separatist activist and his son were shot dead on their farm and gunmen killed a soldier at a police post in worsening violence in southern Yemen, witnesses and media said on Monday.

A fourth man was also shot dead on Sunday by soldiers in Dalea, the centre of southern separatist unrest, media said.

The Sahwa news website said activist Naji bin Naji and his young son Saleh were killed on their qat farm near Dalea on Sunday. One suspect had been arrested.

Another website quoted residents as saying the killer was a member of the local security forces. Clashes between gunmen and security forces broke out north of Dalea after the killings, it said.

Tension between Yemeni security forces and southern secessionists protesting against President Ali Abdullah Saleh's government have been on the rise in recent weeks, accompanied by widespread arrests and deaths on both sides.

North and South Yemen formally united in 1990 but many in the south, where most of impoverished Yemen's oil facilities are located, complain northerners have used unification to seize resources and discriminate against them.

In Dalea, commerce halted as residents responded to a call for a general strike that blocked the transport of narcotic qat plants, which produce a widely used mild stimulant, from Dalea to the main southern city of Aden, the Mareb Press website reported.

Separately in the south, gunmen shot dead a Yemeni soldier standing guard at a police post on Sunday in Shabwa province, local media and witnesses said. – Reuters