US commandos killed a senior Islamic State group leader in a nighttime raid into Syria, American officials said Saturday, as IS jihadists seized the northern part of Syria's ancient desert city of Palmyra.

Across the border, IS battled Iraqi army reinforcements in the strategic western city of Ramadi, while Turkey said its armed forces had shot down a Syrian helicopter that had violated its airspace.

US President Barack Obama approved the special forces raid on Al-Omar in east Syria on Friday night to capture senior IS leader Abu Sayyaf and his wife Umm Sayyaf, the government said.

The bold operation, with elite commandos striking at IS's inner circle, was a rare use of "boots on the ground" by the United States, which has fought the jihadists almost entirely from the air.

White House national security spokeswoman Bernadette Meehan said Abu Sayyaf, who played a senior role in IS's lucrative oil operations, "was killed when he engaged US forces".

His wife was being held in military detention in Iraq while a young Yezidi woman, who appears to have been held as a slave by the couple, has been freed.

Al-Omar, one of the largest oilfields in Syria, is in oil-rich Deir Ezzor province, much of which is controlled by IS extremists.

US Secretary of Defence Ash Carter called the operation a "significant blow" to IS, while Adam Schiff, a Democrat on the House intelligence committee, said US attacks "have put increasing pressure on the economics undergirding the terrorist organization".

Meehan said US forces based out of Iraq conducted the raid "with the full consent" of the Iraqi authorities. They suffered no casualties, officials said, without giving details on the number involved.

Members of the elite Delta special operations unit descended on Sayyaf's compound in Black Hawk helicopters and Osprey tilt-rotor aircraft, a defence official told AFP.

- 'Hand-to-hand combat' -

US troops killed "about a dozen" militants in a gun battle before fighting them "at very close quarters... there was hand-to-hand combat," the official said, speaking on condition of anonymity.

Damascus, which Washington did not say it consulted, said the Syrian army had killed IS's "oil minister" in Al-Omar, naming him as Abu al-Taym al-Saudi. A Syrian military source would not confirm if this was another man or Abu Sayyaf.

News of the raid comes the day after Washington authorised sending weapons to the Iraqi military after IS made key territorial gains, seizing a government compound in the strategic Iraqi city of Ramadi.

Fighters have also taken over the northern neighbourhoods of the Syrian city of Palmyra following an assault on the ancient metropolis that has seen them execute 49 people in two days, a monitoring group said.

"IS advanced and took control of most of northern Palmyra, and there are fierce clashes happening now," said Rami Abdel Rahman, director of the Syrian Observatory for Human Rights.

The head of Syria's antiquities department, Mamoum Abdulkarim, voiced extreme concern for the UNESCO world heritage site located to Palmyra's southwest.

"I am living in a state of terror," Abdulkarim told AFP, adding that IS "will blow everything up. They will destroy everything" if they enter the site.

Fearing the destruction of Palmyra, known as the "pearl of the desert," UNESCO has called on the UN Security Council to act to save one of the Middle East's historic treasures.

- Air raids kill 48 -

Meanwhile in northwest Syria, at least 48 civilians including nine children were killed on Saturday in regime air raids on Idlib province, the Observatory said.

It said the air strikes targeted rebel-held Idlib city and the towns of Saraqeb and Kafr Awid.

Separately, Turkey's defence minister said its armed forces shot down a Syrian helicopter that violated Turkish airspace on Saturday.

"A Syrian helicopter was downed that violated the border for a period of five minutes within a seven mile (11 kilometre) limit," Defence Minister Ismet Yilmaz said, quoted by the Dogan news agency.

Prime Minister Ahmet Davutoglu told Turkish channel Star TV the episode served to show Syrian President Bashar al-Assad that "whoever you are, if you violate our border, you will be punished".

Syrian state television had earlier indicated the aircraft was a drone and vehemently denied it could have been a manned aircraft.

In Iraq, IS tightened its siege on the last government positions in Ramadi, the capital of Anbar province west of Baghdad, a day after they seized the city's government headquarters.

Taking the city would be the group's most important victory this year in Iraq, and would give it control of the capitals of two of its largest regions, along with Mosul in Nineveh province.

Anbar province extends from the Syrian, Jordanian, and Saudi borders to the gates of Baghdad.

Military reinforcements have been sent to Ramadi and other parts of Anbar, local officials said, and Iraq's army and the US-led coalition have struck IS positions in the area.