But Mr Assad condemned the involvement of Britain launching airstrikes in Syria.
'It will be harmful and illegal and it will support terrorism, as happened after the coalition started its operation a year or so [ago] because this is like a cancer,' he told The Sunday Times in an interview carried out on Wednesday.
'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.'
He added: 'You cannot defeat (Daesh) through airstrikes alone. You cannot defeat them without co-operation with forces on the ground.
'You cannot defeat them if you do not have buy-in from the general public and the government . . . . They are going to fail again.'
Mr Cameron faced intense criticism in the Commons last week over his claim of 70,000 moderate fighters on the ground in Syria who could be supported by British airstrikes.
Ministers have been forced to clarify that they are disparate groups who are a long way from being a cogent fighting force.
In a withering attack on the British Prime Minister, Mr Assad added: 'This is a new episode in a long series of David Cameron's classical farce ... Where are they? Where are the 70,000 moderates he is talking about? There is no 70,000. There is no 7,000.'
Britain has been bombing ISIS targets in Iraq at the invitation of the Iraqi government for more than a year.
But after the Paris terror attacks in which 130 people died, French President Francois Hollande appealed for Britain to join the fight in Syria.
Initital strikes have been against the Omar oilfields, but and many of the airstrikes will be carried out at night to try to limit the risk of civilian casualties.
But Mr Fallon warned they could not be avoided altogether.
'War is a messy business; you cannot eliminate all risk,' he told the Sunday Times.
'It is very important for the campaign that we avoid mistakes. You can't completely eliminate mistakes in wartime, but we do our best to ensure that any civilian damage is minimised.
'I also have to be sure that what's being proposed is absolutely consistent with the rules of engagement that I set and that the prime minister has approved.'
The Russian airforce has struck nearly 1,500 targets all over Syria over the past nine days, the military said, its bombers now flying under cover of strategic fighter jets following the downing of a plane by Turkey last week.
Defence ministry spokesman Igor Konashenkov said in a briefing that the Russian air contingent carried out 431 sorties and bombed 1458 targets in several regions of Syria, without mentioning whether any of them were affiliated with the Islamic State jihadist group.
Among the targets was a 'command post' near the town of Khnaifess in the Homs region, and a 'large ammunitions stockpile' near Morek in Hama region, which was captured from the Syrian army in early November.
The airstrikes also 'destroyed a large militant base' at a strategic location near Kassab in Latakia region, leading to Syrian army 'taking the high ground,' the ministry said.
In Hama region, the airstrikes 'liquidated' several field commanders near rebel-held Latamina, and bombed several strategic high-ground locations, including Hazm al-Abyad.
By Matt Chorley
