More than 30 civilians have been killed and 150 injured in air strikes targeting areas held by the Islamic State of Iraq and the Levant group (ISIL) in Syria, an activist group has said.

Raqqa is Being Slaughtered Silently group said on Wednesday that 32 civilians were killed and 150 injured in Russian strikes on Raqqa city, the de-facto capital of ISIL.

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The Britain-based Syrian Observatory for Human Rights, which relies on a network of activists inside Syria, put the death toll at 25 civilians, including six children.

The raids came a day after ISIL, also known as ISIS, pushed government forces some 40km from the strategic city of Tabqa, an area west of Raqqa city that has Syria's largest dam and an airbase.

Government troops and allied militiamen had reached within 7km of the airport on Sunday, according to the Observatory.

Tabqa airbase was the last position held by government forces in Raqqa province before ISIL overran it in August 2014, killing scores of detained soldiers in a massacre they documented on video.

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ISIL has imposed strict rule in Raqqa city - home to more than 220,000 people before the Syrian conflict - and committed atrocities against the civilian population since its takeover more than two years ago.

The group has been under pressure in Iraq, Syria and Libya in recent weeks, but the recent gains in Raqqa show it is still able to take on Syrian troops backed by Russian warplanes.

But according to defence analysts at the think-tank IHS Jane, ISIL has lost about 14 percent of its territory in 2015 and is struggling to hold on to territory under bombardment from the coalition and having to fend off rival armies and factions on multiple fronts.

The Syrian civil war started as a largely unarmed uprising against President Bashar al-Assad in March 2011, but quickly developed into a full-on armed conflict.

United Nations special envoy for Syria, Staffan de Mistura, estimated in April that more than 400,000 Syrians had been killed, though he said that number was not an official UN statistic.

Almost 11 million Syrians - half the country's pre-war population - have been displaced from their homes.

Source: Al Jazeera