BEIRUT (Reuters) - Gunmen assassinated a well-known media defender of Syrian President Bashar al-Assad in Lebanon on Wednesday, as Syria's civil war steadily infects its smaller neighbour.

The conflict is also spilling over Syria's northern border, where Turkish troops returned fire after stray bullets from Syria struck the police headquarters and several homes in the Turkish border town of Ceylanpinar, the Turkish military said.

In Lebanon, Mr Mohammad Darra Jamo, a commentator for Syrian state media who often appeared on Arab TV channels to promote Mr Assad's cause, was killed by gunmen at his home in the southern Shi'ite Muslim town of Sarafand, security sources said.

It was the first assassination of a pro-Assad figure in Lebanon since the Syrian revolt began 28 months ago, and follows several attacks in recent weeks against the Lebanese Shi'ite Hizbollah group, which is now fighting for Mr Assad in Syria.

Lebanon, whose own 15-year civil war ended in 1990, is struggling to stay on the sidelines of the conflict next door, but car bombs and clashes between groups supporting rival sides in Syria have become increasingly common.

Syrian state media blamed an "armed terrorist group" for the killing, which took place at around 11pm GMT on Tuesday, and the Syrian Information Ministry called it a "heinous crime".

Lebanese security sources said supporters of Syrian rebels were top suspects in the killing in a Shi'ite area of the south, where Hizbollah's security control is normally tight.

Mr Jamo's wife, who was with him when he was shot but was not hurt, said on Hezbollah's al-Manar television channel that officials from Syria's ruling Baath party had called her husband on Tuesday and warned him to be careful.

The same day a Hizbollah security official was killed and two were wounded when a convoy of vehicles heading to Syria from Lebanon was ambushed with a bomb and gunfire.

A Syrian Kurd who had lived in Sarafand for 20 years, Mr Jamo appeared regularly on al-Manar television and radio broadcasts.

Syria's mostly Sunni Muslim rebels have threatened to take the war into Lebanon after Hizbollah sent fighters to help Mr Assad, whose Alawite sect is a branch of Shi'ite Islam.

The war in Syria, which has cost more than 94,000 lives, has taken on an increasingly sectarian and fragmented aspect.

For several weeks, pro-Assad forces have been making military gains in Syria, while rebel groups have been plagued by infighting between Islamist militants and the Free Syrian Army.