BEIRUT/LATAKIA, Syria: At least four people were killed in a blast in the Syrian government stronghold of Latakia on Monday, but there were conflicting reports about what caused the explosion.

It comes amid heightened fears of attacks in the coastal region after a rebel alliance that includes the Al-Qaeda-affiliated Nusra Front last month seized Jisr al-Shughur in neighboring Idlib province.

The Syrian Observatory for Human Rights and sources on the ground both reported the blast in the city of Latakia in the coastal province of the same name.

An AFP correspondent in the city saw flames and a column of smoke rising after hearing what appeared to be an explosion in the Mar Taqla neighborhood.

At the scene of the blast, residents said those killed were a father and his son and daughter, along with their female neighbor.

The explosion set fire to the family's apartment and partially collapsed the building's roof.

The Britain-based Observatory also reported that the blast had killed four people, among them two children.

But there were differing accounts of what caused with the blast, with officials in the city refusing to comment and residents saying the cause was unclear.

The Observatory said the blast was the result of a drone that crashed into the city after being hit by a rocket fired from a ship offshore in the Mediterranean.

But the group said it was unclear whose drone it was.

Sources on the ground meanwhile said the blast appeared to be caused by rocket fire, though rebel forces are not known to be close enough to the city to fire rockets into it.

On social media, photos and video purporting to be from the incident showed a column of orange smoke rising into the air.

Latakia province is a bastion of President Bashar Assad's government and home to his ancestral village Qardaha.

The western seaboard region of Syria has largely been spared the worst of the violence that has wracked the country since an uprising that began in March 2011.

Many Syrians displaced by violence in neighboring regions have taken refuge in Latakia province, and entrepreneurs have also moved business to the relative safety of the area.

Meanwhile Monday, Syrian government aircraft carried out intense strikes against ISIS in and around the ancient city of Palmyra after its fall to the jihadis, a military source said.

"The air force struck more than 160 ISIS targets, killing and wounding terrorists and destroying weapons and vehicles equipped with machineguns" on Palmyra's outskirts and elsewhere in the east of Homs province, the source said.

"Military operations, including air raids, are ongoing in the area around Al-Suknah, Palmyra, the Arak and Al-Hail gas fields and all the roads leading to Palmyra," he said.

State television said "more than 50 ISIS terrorists" had been killed in the airstrikes.

The Syrian Observatory for Human Rights said at least four civilians had been killed in the raids, which were the most intense since the jihadis overran the city on Thursday.

Dozens of people had also been wounded in the raids, and ISIS was believed to have taken losses when a military security building was hit, said director Rami Abdel Rahman.

The strikes targeted several areas of the city, including some close to the city's famed Greco-Roman ruins, a UNESCO world heritage site, he said.

But so far they had failed to halt the jihadis, who advanced towards the capital Damascus and overran major phosphate mines about 70 kilometers (45 miles) south of Palmyra.

"[ISIS] has made further progress on the Tadmor-Damascus highway and grabbed the Khnaifess phosphate mines and nearby houses," said the Observatory, which relies on a network of sources on the group for its reports.

"It has extended its control over larger areas and even greater economic interests," added the Observatory.

The latest mines seized by ISIS are the second largest in the country.

In the first half of 2014, the General Company for Phosphate and Mines reported sales of $30 million on production of almost 500,000 tons, down from two million tons in 2011.

Syria is considered to have one of the world's largest phosphate reserves.

"With the suspension of oil exports, phosphates represented one of the last sources of income of the state," according to Syria Report, an online business weekly.

ISIS is accused of executing hundreds of people in and around Palmyra since it swept into the oasis city last week after a lightning advance across the desert from its stronghold in the Euphrates Valley to the east.

The Observatory said on Sunday that it had documented the executions of at least 217 people, among them 67 civilians, including 14 children.

Some of those killed had been beheaded, Abdel Rahman said, adding that the jihadists had also taken some 600 people prisoner.

Syrian state media said at least 400 civilians had been killed by ISIS in Palmyra, most of them women, children and old men.

The pro-government Al-Watan daily reported on Monday that the number of executions had risen to 450.