The Syrian army launched airstrikes on Monday against the Islamic State (IS) in the ancient city of Palmyra, where the terrorist group are reported to have killed hundreds of people since taking over the UNESCO World Heritage site last week.

Buildings occupied by the militants were targeted in at least 15 air strikes which killed four civilians and injured dozens of people, according to the British-based Syrian Observatory for Human Rights (SOHR). A Syrian military official confirmed the strikes to AFP, saying: "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."

IS is reported by Syrian state media to have killed more than 400 soldiers and civilians, including women and children, since taking control of the city. Activists said on social media that hundreds of beheaded corpses were lying in the streets, reported Reuters.

The SOHR has said more than 300 soldiers were killed in the days of fighting before the city was taken. It estimates the militants now control 50 percent of Syrian territory.

It also said Sunday it had documented an additional 150 people killed by IS for being "informers for the regime forces" — the majority of whom it said had been beheaded. More than 600 members of the Syrian regime's forces and their allies were being detained, and faced probable execution, it added.

State-run news agency SANA said IS was preventing people leaving Palmyra, reported the Associated Press, while the UN refugee agency UNCHR said it was preparing aid for the 11,000 people who had managed to flee.

On Sunday, Governor Talal Barazi of Homs — a province which includes Palmyra — told AP that IS fighters had "committed mass massacres in the city of Palmyra" since they gained control of the city. He also said that many civilians — including women — had been taken to unknown destinations.

The official added that the Syrian army troops were being deployed in areas near to the city in preparation for a counterattack.

IS has released images showing its flag flying in Palmyra's ancient city, heightening fears the group is planning to destroy the ruins — some of the most spectacular and best-preserved in the world. IS believes ancient relics promote idolatry and has destroyed cultural heritage sites dating back thousands of years in Iraq.

An activist in Palmyra posted this video, which he claims shows the aftermath of air strikes in the city by the Syrian Air Force

Meanwhile in an interview with the BBC on Monday, Iraqi Prime Minister Haider al-Abadi rejected accusations that his country's army had failed to even try to protect the city of Ramadi, which was also taken by IS last week. Ramadi is the capital of Iraq's largest province, Anbar province, and lies just 70 miles from Baghdad.

In a interview aired on Sunday US Defense Secretary Ashton Carter blamed Iraq's weak military for the the loss of the city.

"What apparently happened was that the Iraqi forces just showed no will to fight," Carter told CNN. "They were not outnumbered. In fact, they vastly outnumbered the opposing force, and yet they failed to fight, they withdrew from the site, and that says to me, and I think to most of us, that we have an issue with the will of the Iraqis to fight [IS] and defend themselves."

Al-Abadi told the BBC he was surprised by Carter's comments and the defense secretary must have been given misinformation. Iraqi forces had been overcome by IS, he claimed.

"They have the will to fight but when they are faced with an onslaught by [IS] from nowhere... with armoured trucks packed with explosives, the effect of them is like a small nuclear bomb - it gives a very very bad effect on our forces," he said.

The Iraqi leader said Shia militias had been deployed by government and he was confident Ramadi could be recaptured "in days."

When asked about the perception that IS are gaining major ground, al-Abadi said: "They only won this battle, it doesn't mean they're winning. When they concentrated their efforts on many fronts and they failed on all of them [except] this one, that's not winning. If they only won this tactical battle they want to send this message that they are winning, but they're not. I think if you compare our situation now to last summer it's completely different."

"We have decided not to fight into the cities," al-Abadi added. "In the open you can control the area and you can push them back."