US aircraft have launched a fresh wave of bombing raids against the Islamic State of Iraq and the Levant (ISIL) in Syria, in the fifth consecutive day of air strikes in the country, according to a Pentagon official.

The US military's Central Command, which leads American forces in the Middle East, did not issue any details of the latest assault, but the defence official, speaking to AFP news agency on Friday, said: "I can confirm US air operations are ongoing in Syria."

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

The news came as Britain, Belgium and Denmark promised at least 19 fighter jets to join the US-led aerial offensive against ISIL, freeing the US to strike "jihadist targets" in Syria.

Earlier, Chuck Hagel, US defence secretary, said US and Arab coalition warplanes carried out more than 40 bombing strikes this week against the ISIL, including attacks on the organisation's oil refineries.

Activists also confirmed the raids on oil installations and other facilities in ISIL territory in eastern Syria.

The air strikes reportedly hit two oil areas in Deir Az Zor province on Friday, a day after the US and its Arab allies attacked a dozen makeshift oil-producing facilities in the same area near Syria's border with Iraq.

The US Central Command said three air strikes south and southeast of Deir Az Zor destroyed four ISIL tanks and damaged another.

Just right across the border in Iraq, the latest air attacks against ISIL have reportedly killed 52 of its fighters, sources told Al Jazeera.

ISIL fighters were bombed in the town of al-Qaim, and it was said that they were trying to cross the border into Syria when they were hit.

In another development, the US is planning to train and arm 5,000 Syrian rebels, 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".

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