US-led coalition warplanes have destroyed four tanks belonging to the Islamic State (IS) group in eastern Syria during action in a key oil-producing area that has generated millions of dollars for the extremist faction.

The strikes outside the city of Deir el-Zour on the Euphrates River marked the second consecutive day that the United States and its Arab allies have taken aim at the militants near the border with Iraq.

Coalition planes pounded a dozen makeshift oil-producing facilities in the same area on Thursday, trying to cripple one of the militants' primary sources of cash - black market oil sales that the US says produce up to $2 million a day.

The British-based Syrian Observatory for Human Rights said the American-led air campaign also hit the Tanak oil field as well as the Qouriyeh oil-producing area in Deir el-Zour today.

It added that air raids also targeted the headquarters of IS in the town of Mayadeen south-east of Deir el-Zour city.

Activists added that the strikes caused an unknown number of casualties, and were believed to have been carried out by the coalition.

Another activist collective, the Local Coordination Committees, also reported four strikes on Mayadeen that it said were conducted by the US and its allies.

The US Central Command confirmed that US aircraft destroyed four Islamic State group tanks and damaged another.

It did not have any information on strikes targeting oil facilities, but the military statement mentions raids by American forces only and not those by other coalition partners.

The Observatory reported another apparent coalition air raid on IS positions outside the city of Hassakeh in north-eastern Syria near the Iraqi border.

Those strikes targeted an oil-production area, as well as vehicles the militants had brought in from Iraq and tried to bury in the ground to protect them, according to Observatory director Rami Abdurrahman.

The coalition, which began its aerial campaign against Islamic State fighters in Syria early on Tuesday, aims to roll back and ultimately crush the extremist group, which has created a proto-state spanning the Syria-Iraq border.

Along the way, the militants have massacred captured Syrian and Iraqi troops, terrorised minorities in both countries and beheaded two American journalists and a British aid worker.

The air assault has targeted Islamic State checkpoints, training grounds, oil fields, vehicles and bases as well as buildings used as headquarters and offices.

Activists say the militants have cut back the number of gunmen manning checkpoints, apparently fearing more strikes. There has also been an exodus of civilians from Islamic State strongholds.

In towns and villages controlled by Syria's mainstream rebel factions, the air strikes have garnered mixed reactions. Most people appear to condone hitting IS, but question why president Bashar Assad's forces - which have killed thousands of people in the civil war - remain untouched.

Syrians are also critical of the US decision to bomb the al-Qaida-affiliated Nusra Front in the opening salvo.

The Nusra Front enjoys a degree of support among many in the opposition because its fighters are on the front lines against Assad.

In several opposition-held areas Friday, residents staged demonstrations condemning the air strikes, according to activists and videos posted online.

One amateur video posted online shows people in the village of Taftanaz in Idlib province denouncing the air strikes. One placard in English read: "What about the regime's terrorism?"

Another video shows a similar protest in the village of Iblin.

The coalition campaign in Syria expands upon the air strikes the US has been conducting on its own against the extremist faction for more than a month in Iraq. France joined the American effort in Iraq a week ago, and is considering whether to extend its air strikes to Syria as well.

Denmark, meanwhile, announced Friday it would send seven F-16 fighter jets to take part in the air strikes in Iraq, two days after the Netherlands did so. Belgium has said it will send six F-16 fighter planes to take part in the operation.The European countries do not plan to deploy in Syria.

The international operation targeting the Islamic State group adds another layer to Syria's civil war, a conflict that has already killed more than 190,000 people since the revolt against Assad began in March 2011.

While overshadowed by the coalition strikes against the Islamic State, fighting between Syrian government troops and rebels has raged on with its usual ferocity.