* Yemen separatist, young son killed on their qat farm
* Yemen soldier shot dead at police post in south
* Yemeni killed by security patrol near Dalea
ADEN, Yemen, April 5 (Reuters) - 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.
Yemen became a Western security concern after the Yemen-based regional arm of al Qaeda claimed responsibility for a failed attempt to bomb a U.S.-bound plane in December.
Western allies and neighbouring oil exporter Saudi Arabia fear al Qaeda is exploiting Yemen's instability to recruit and train militants for attacks in the region and beyond. In addition to its struggle with the separatists, Yemen is battling Shi'ite rebels in the north. It sealed a truce with the rebels in February but analysts say it is only a matter of time before fighting starts again as grievances have not been adequately addressed. (Reporting by Mohamed Mukhashaf; Writing by Cynthia Johnston: Editing by Angus MacSwan)