The assassination of a third Syrian journalist in Turkey in recent days is being blamed on Islamic State militants, with the latest killing raising deep concerns about the extent of the group's penetration into the country.

In broad daylight last Sunday, in the southern Turkish city of Gaziantep, 37-year-old filmmaker Naji Jerf was gunned down by a masked man. At the time, he was buying lunch for his family, accompanied by two friends. He was shot twice at close range in the face and torso.

The murder has sent Syrian activists and journalists in Turkey into hiding and prompted promises of asylum from an unnamed European country.

Jerf, the father of two, was a well known and popular reporter who had documented IS abuses and trained citizen journalists with the activist group, Raqqa is Being Slaughtered Silently (RBSS).

Syrian journalist and activist Ibrahim Abdelqader and a colleague were found partially decapitated in Turkey in October in an apparent double assassination. Islamic State is being blamed for the killings. ( Supplied )

The group documents IS crimes against civilians in Syria. Jerf had just released his new documentary, ISIS in Aleppo, about Syrian activists killed by IS in that city.

He was also the director of Hentah, an independent monthly magazine supporting elements of the Syrian opposition.

His death comes just two months after two other Syrian journalist/activists, Ibrahim Abdelqader and Fares Hamadi, were stabbed to death and partially decapitated in the Turkish city of Urfa, an hour east of Gaziantep.

The pair had been active with another anti-IS group, Eye on the Homeland.

IS sleeper cells suspected in Turkish territory

As IS took territory in Syria, killing at least nine journalists, many media workers and civil rights campaigners took refuge in the relative safety of Turkey.

Gaziantep and Urfa, just 40 kilometres from the Syrian border and close to the IS capital, Raqqa, have become hubs for NGOs, aid workers and journalists covering the conflict in Syria.

Naji Jerf's burial in the Turkish city of Gaziantep this week after his apparent assassination. ( Supplied )

IS sleeper cells have long been suspected of operating inside Turkish territory and the recent audacious murder of three journalists on Turkish soil has reinforced a clear message: IS enemies are not safe, even across borders.

Interviewed just two months before he was killed in Urfa with his colleague, Abdelqader told the ABC he had been threatened multiple times, but continued to document IS crimes and assist defectors.

Investigations into the murders suggest the two men were killed by an associate posing as an IS defector who had spent months cultivating a friendship, before fleeing back to Syria after the murders.

Other Syrians in Urfa have refused to be interviewed by western journalists, fearing retribution from IS members they say are definitely operating in Turkey.

A martyrs funeral was held for Jerf in Gaziantep on Monday amid heightened security. Many of the 100 mourners sang revolutionary songs and chanted: "We are all Naji."

"I was burying my friends in Syria, now I am burying them in Turkey," said Mazen Gharibah, a Syrian opposition activist and close friend of Jerf, who was at the hospital when he died.

While the outpouring of grief and support for Jerf has been loud, activists are now making plans to leave the country. The ABC understands all those affiliated with RBSS have been offered asylum in a European country.

"All the activists are really afraid," Mr Gharibah said.

"Naji had released his film just 10 days ago ... we all know that's why he was killed."

Calls for greater security for media

In the last conversation with his friend just days before his murder, Jerf told Mr Gharibah he had been threatened and planned to leave Gaziantep for Istanbul to celebrate his daughter's birthday, before travelling to France this week, where he had recently been granted asylum.

"He was really courageous. He cut his hair and changed his appearance, but IS must have known he was leaving," Mr Gharibah said.

Mourners at the funeral for Naji Jerf. ( Supplied )

The Committee to Protect Journalists (CPJ) has called on the Turkish government to provide greater security for media workers and for authorities to swiftly find his killers. An investigation into the three deaths is underway, but so far no one has been arrested.

CPJ Middle East and North Africa co-ordinator, Sherif Mansour, said the assassinations, and the escape of Abdelqader's and Hamadi's killer to Syria, were carefully planned using sophisticated intelligence that must have been coordinated with operatives on Turkish soil.

"Syrian journalists who have fled to Turkey for their safety are not safe at all," Mr Mansour said in a statement.

"But if journalists decide not to cover these stories because it is too dangerous, then the stories won't be told and the terrorists win."