US aircraft have launched a fresh wave of bombing raids against Islamic State militants in Syria, in the fifth consecutive day of air strikes in the country, a defense official told AFP on Friday.

"I can confirm US air operations are ongoing in Syria," said the senior official, who spoke on condition of anonymity.

The mission is now similar to US-led air raids undertaken in Iraq, with "near continuous" combat flight operations over Syria, the official said.

The US military's Central Command, which leads American forces in the Middle East, did not issue any details of the latest air strikes.

Pentagon chief Chuck Hagel told reporters earlier Friday that US and Arab coalition warplanes have carried out more than 40 bombing strikes this week against the IS group, including attacks on the organization's oil refineries.

"Combined with our ongoing efforts in Iraq, these strikes will continue to deny ISIL (IS group) freedom of movement and challenge its ability to plan, direct, and sustain its operations," Hagel said.

Anti-jihadist coalition widens as strikes hit Syria oil assets
London (AFP) Sept 26, 2014 - The US-led coalition against the Islamic State group widened on Friday with Britain, Belgium and Denmark approving plans to join air strikes in Iraq.

But Washington said up to 15,000 "moderate" rebels would be needed to beat back the jihadists in Syria, where the Pentagon said air raids had disrupted lucrative oil-pumping operations that have helped fund the militants.

The White House welcomed the new countries recruited to the Iraq operation, who are expected to deploy a total of 19 fighter jets in the air campaign.

Britain's House of Commons voted overwhelmingly by 524 lawmakers in favour and 43 against a motion authorising air strikes in Iraq.

Defence Secretary Michael Fallon said there would be no "immediate military action" but that it would be a "long, drawn-out campaign".

"We have to select our targets in accordance with the American and international effort that's going on in Iraq," Fallon told Sky News, also noting there had been "a lot of support" for military action in Syria during a sometimes heated parliamentary debate.

Ahead of the vote, Prime Minister David Cameron told lawmakers IS must be confronted.

"This is not a threat on the far side of the world. Left unchecked, we will face a terrorist caliphate on the shores of the Mediterranean," Cameron said.

Washington is eager to build the broadest possible coalition including Arab allies to tackle IS, which has captured large areas of Syria and Iraq and declared an Islamic "caliphate".

- Oil trade shutdown -

Britain, Belgium and Denmark should join warplanes from the United States, France, Saudi Arabia, the United Arab Emirates, Bahrain and Jordan that are already hitting IS targets.

The Netherlands is also sending six F-16 jets and will provide 250 military personnel and 130 trainers for the Iraqi military, and Greece said it would send arms to Kurdish forces in Iraq.

In recent days, Washington and its allies have targeted the funding sources of what US President Barack Obama has branded a "network of death".

In the latest air strikes, US planes destroyed four tanks operated by militants in Syria as well as several vehicles and jihadist positions in Iraq, the Pentagon said.

The US-led coalition also bombed oil refineries in east and northeast Syria where IS jihadists extract crude for sale on the black market, according to the Syrian Observatory for Human Rights, a British-based monitoring group.

Experts say sales of oil from Syria and Iraq usually earn IS up to $3 million (2.4 million euros) a day.

But now, according to activists in Deir Ezzor, pumping has stopped.

"Oil extraction has been halted because of the security situation," said Leith al-Deiri, who spoke to AFP via the Internet.

Another activist from Deir Ezzor, Rayan al-Furati, confirmed the halt.

"There are no traders or clients going to the fields, (because they are) fearing the strikes," he said.

- 'Ground component' -

The US is also planning to train and arm 5,000 Syrian rebels as part of the effort, although the top US military officer, General Martin Dempsey, said a force of between 12,000 and 15,000 would be required "to recapture lost territory in eastern Syria".

The general said defeating the IS group would take more than air power and that "a ground component" was an important aspect of the US-led campaign.

"We believe the path to develop that is the Syrian moderate opposition," he said.

Britain and France have both for now ruled out launching strikes in Syria at the hub of the IS group's power, unlike the Arab allies taking part in the air campaign.

London however said it would "reserve the right" to intervene in Syria in case of a "humanitarian catastrophe", for example in the case of an imminent massacre, without first consulting parliament.

IS's brutal abuses against civilians, rival fighters and Arab and Western hostages, as well as its success in recruiting Western members, have caused international alarm.

British police this week arrested 11 people suspected of links to Islamic extremists, including a notorious radical preacher who was released on Friday.

In France, hundreds of people gathered in Paris answering a call by Muslim leaders to denounce the "barbarism" of Islamic State, as flags across France flew at half mast after the beheading of a French national by an IS-linked group in Algeria.

The coalition strikes in Syria are reported to have killed at least 140 jihadists as well as 13 civilians.

The Syrian Observatory on Friday reported that a senior IS jihadist had been killed with a targeted strike on the motorbike he was travelling in after leaving a military base in eastern Syria.

The conflict that began in Syria in March 2011 as an uprising against President Bashar al-Assad's regime has spawned a massive refugee crisis, with more than three million Syrians now taking refuge from the war abroad.