Syrian government aircraft carried out intense strikes Monday against the Islamic State group in and around the ancient city of Palmyra after its fall to the jihadists, a military source said.
"The air force struck more than 160 Daesh 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.
"We are pursuing Daesh wherever they are," the source said, using the Arabic acronym for IS.
"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 Daesh terrorists" had been killed in the air strikes.
The Syrian Observatory for Human Rights said at least four civilians had been killed in the raids, which were the most intense since the jihadists overran the city on Thursday.
Dozens of people had also been wounded in the raids, and IS was believed to have taken losses when a military security building was hit, said Observatory 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 jihadists, who advanced towards the capital Damascus and overran major phosphate mines about 70 kilometres (45 miles) south of Palmyra.
- IS seizes key mines -
Signup to our newsletter and follow us on Facebook and Twitter!
"IS 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 monitor.
The latest mines seized by IS 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 tonnes, down from two million tonnes 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.
IS 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 IS 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.