Backed forces 'capture' Tabqa airbase from ISIL

US-backed forces battled the militant Islamic State group around a key Syrian town on Monday, after the capture of an airbase brought them closer to besieging the jihadists in their stronghold Raqa.

The attack also included an unprecedented air assault involving U.S. helicopters landing behind enemy lines - flying about 500 local USA allies and coalition military advisers across the Euphrates River and Lake Assad so they could attack the ISIS-controlled dam and neighboring town and airfield from the south. Earlier this month, USA aircraft ferried hundreds of fighters from the Kurdish-led Syrian Democratic Forces, as well as American artillery and military advisers, behind IS lines.

The United Nations warned this year of catastrophic flooding in Syria from the Tabqa dam, which is at risk from high water levels, deliberate sabotage by Islamic State and further damage from air strikes by the USA -led coalition.

We review the key developments in Syria, including United Nations special envoy Staffan de Mistura not expecting any miracles in peace talks, USA -backed forces taking control of the Tabqa airbase and airstrikes on a prison in Idlib killing 16 people.

The Taqba dam, also known as the Euphrates dam, is seen as a prize to push ISIL out of Raqqa, the armed group's self-proclaimed capital in Syria.

Raqqa 24 said engineers employed by the militants had restored power to the dam's gates and the structure was functioning normally.

After a brief pause in fighting on Monday to allow technicians to enter the dam complex, SDF fighters resumed their operations around the structure, said spokeswoman Jihan Sheikh Ahmed.

"This time, coalition forces took part in greater force than previous occasions with new tactics like the airlift, crossing the river, and artillery fire", she said.

Activist groups said Monday the city itself has been without electricity and municipal water for three to five days.

There are hundreds of US special forces, Marines and army rangers in northern Syria.

As with thousands of civilians in the besieged northern Iraqi city of Mosul, residents of Raqqa remain trapped between the rule of ISIS, the incoming militias fighting to liberate the city and the airstrikes of the US -led coalition.

The engineer Ahmad Farhat, who oversaw the mechanical administration of the dam, said that it is "equipped with the necessary precautions for its own protection", but there needs to be technical personnel on site to engage them.

The Syrian Observatory and the activist-run Raqqa 24 media centre reported that as of Sunday there were no evacuations in Raqqa.

However, civilian rights groups have expressed safety concerns for citizens still living in the area.

"We do not assess the dam to be in imminent danger unless ISIS plans to destroy it", the rebel group said in a statement posted by the Combined Joint Task Force.

Islamic State fighters captured the Tabqa airbase from the Syrian Arab Army (SAA) in August 2014, during the group's sweep through wide swathes of Iraq and Syria.

Rami Abdel Rahman, head of the Syrian Observatory for Human Rights monitor, said the alliance was fighting north of the airport to reinforce its positions. The exiled opposition coalition is taking part in UN-mediated talks in Geneva.