The number of Europeans joining Islamist fighters in Syria and Iraq has risen to more than 3,000, the EU's anti-terrorism chief has told the BBC.

Gilles de Kerchove also warned that Western air strikes would increase the risk of retaliatory attacks in Europe.

US-led forces have launched nearly 200 air strikes against Islamic State (IS) militants in Iraq since August and on Monday began targeting IS in Syria.

The UK parliament is due to vote on possible air strikes in Iraq on Friday.

IS has seized large parts of Iraq and Syria in recent months.

Mr de Kerchove said the number of 3,000 included all those who have been to the region, including those who have returned and those who have been killed there.

Earlier this year, experts put the figure at about 2,000.

Mr de Kerchove said that Islamic State's declaration of a Caliphate in June may have played a role in drawing more support from Europe.

"If you believe in this, probably you want to be part of it as early as possible," he said.

He warned that air strikes by the US and its Western allies had increased the risk of a violent response from militant Islamists against European targets.

"I think we have to acknowledge that it will," he said.

"That was clear with the French because three days ago Isil [IS] issued a statement saying there would be retaliation against the coalition. A French man was kidnapped in Algeria and he has been beheaded. So they did what they announced."

Mr de Kerchove also warned that groups competing with IS, such as al-Qaeda, may try to launch attacks in Europe to maintain their profile.

"The rise of Isil may prompt al-Qaeda to do something to show that it is still relevant," he said.

On Wednesday, the UN Security Council adopted a binding resolution compelling states to prevent their nationals joining jihadists in Iraq and Syria.

On Thursday, the US military released footage and photographs of its air strikes on oil refineries controlled by IS militants in eastern Syria.

The raids, carried out by US, Saudi and UAE aircraft, targeted 12 refineries in Syria on a third night of air strikes against the militants.

Sales of smuggled crude oil have helped finance the jihadists' offensive in both countries.

Pentagon spokesman Rear Admiral John Kirby said the purpose of the strikes was "not necessarily to kill militants" but to destroy the facilities, which were funding IS through the black market.

He said the strikes had been effective but not yet decisive.

"Even after the hits they've taken, and they have been hit, they still have financing at their fingertips," Rear Adm Kirby added.

"They still have plenty of volunteers. They still have plenty of weapons and vehicles and the ability to move around."

He said it was too soon to talk of "winning" against IS.

The US says more than 40 countries have offered to join the anti-IS coalition.

Later on Friday, UK MPs will be asked to support strikes against IS militants in Iraq - but not in Syria.

The office of Prime Minister David Cameron said UK forces could legally join the bombing of targets in Iraq after the Iraqi minister of foreign affairs wrote to the UN seeking international assistance.

Mr Cameron told the UN on Wednesday IS was an "evil against which the whole world must unite".

(BBC)