London (AFP) - Britain's bombing campaign against Islamic State extremists in Syria is "illegal" and will only cause "terrorism" to spread, President Bashar al-Assad said in an interview published Sunday.

"It will be harmful and illegal and it will support terrorism as happened after the coalition started its operation a year or so (ago)," he told The Sunday Times after British MPs voted on December 2 to join the US-led bombing campaign over Syria.

Terror, he said, was like a cancer which needed to be tackled with a "comprehensive" strategy which would involve working with troops on the ground.

"You cannot cut out part of the cancer. You have to extract it. This kind of operation is like cutting out part of the cancer. That will make it spread in the body faster.

"You cannot defeat (IS) through air strikes alone. You cannot defeat them without cooperation with forces on the ground. You cannot defeat them if you do not have buy-in from the general public and the government," he said.

"They are going to fail again."

- Cameron's 'classical farce' -

Britain began its bombing campaign early on Thursday, hitting an oil field held by IS just hours after a decisive parliamentary vote authorised air strikes.

Momentum to join the air campaign grew after IS militants claimed a deadly series of attacks on Paris last month which killed 130 people and wounded more than 350.

In arguing in favour of the strikes, Prime Minister David Cameron's claimed there were 70,000 moderate Syrian forces on the ground who could help secure territory cleared by air strikes, prompting ridicule from Assad, who denounced it as "classical farce".

"Where are the 70,000 moderates he is talking about? There is no 70,000. There is no 7,000," he said.

In late September, Russia began its own bombing campaign in Syria in support of Assad over a year after a US-led coalition began its strikes targeting the IS group.

Russia is coordinating its air strikes with Damascus, unlike the US-led coalition, whose action has been criticised by Assad and his government as ineffectual. Assad said the Russians had entered the conflict in a "legal way" -- with Syria's permission.

While the western bombing campaign has fuelled extremism, Assad believes that two months of Russian air strikes with ground support from the Syrian army has actually damaged the Islamists' capabilities.

More than 250,000 people have been killed since the Syrian conflict erupted in March 2011 with protests against Assad's regime.