Syrian army deserters killed eight people in an attack on an intelligence building in the north of the country, an opposition group said on Friday.

It said the attack took place on Thursday in Idlib province, between the towns of Jisr al-Shughour and the Mediterranean city of Latakia.

"A group of army defectors ... attacked the Air Force Intelligence center," the British-based Syrian Observatory for Human Rights said. "A clash ensued for three hours which lead to the death of at least eight members of the Air Force Intelligence."

The Observatory and other activists also said at least 20 civilians were killed by Syrian security forces across the country on Thursday, mainly in the provinces of Hama and Homs.

UN High Commissioner for Human Rights Navi Pillay told a news conference on Thursday that Syria is now in a state of civil war with more than 4,000 dead and increasing numbers of defecting soldiers taking up arms against the government of President Bashar Assad.

 "We are placing the figure at 4,000, but really the reliable information coming to us is that it is much more than that," she said.