France and Britain have dismissed suggestions of restoring relations with Syrian President Bashar al-Assad, saying such a move would push moderates into the arms of radical Islamist groups.

With the rise of Islamic State insurgents, some European |Union member states are critical of the position in Paris and London and say it might be time to re-establish communication with Damascus given that a four-year-old revolt has failed to overthrow Assad.

In a column published in Arabic daily Al-Hayat and France's Le Monde, the French and British foreign ministers hit back at those who sought a rapprochement with Assad by saying he was using the fear of Islamic State, which has seized wide areas of northern and eastern Syria, to win back international support.

"Some seem sensitive to this argument," Laurent Fabius and Philip Hammond wrote. "In reality, Bashar represents injustice, chaos and terror. We, France and Britain, say no to all three."

There have also been calls from some politicians and former officials in both countries for a new strategy. This week a four-man cross-party delegation of French parliamentarians travelled to Syria and some met with Assad, and triggering a national debate on the issue.

Former British Army chief Lord Dannant has previously said countries would have to work with Assad to defeat Islamic State, while the UN envoy for Syria Staffan de Mistura said earlier this month that Assad must be part of the solution for defusing the conflict in Syria.

"After 220,000 deaths and millions displaced, it is illusory to imagine that a majority of Syrians would accept to be ruled by the one who torments them," Fabius and Hammond wrote.

"To end their hopes of a better future in a Syria without Assad would be to radicalise even more Syrians, push moderates towards extremism and consolidate a jihadist bastion in Syria."

An initially peaceful street uprising against Assad spiralled into a civil war that has seen a level of suffering some diplomats say justify reestablishing contacts with Damascus in pursuit of a political solution.

Britain and France see Assad's departure as a precondition of peace negotiations but the collapse of his government has become less likely as the war grinds on inconclusively.

"For our own security, we must defeat Islamic State in Syria. We need a partner that can act against extremists. We need a negotiated political settlement," Hammond and Fabius said, and a compromise would be needed between elements of the existing government and relatively moderate opponents of Assad.

Reuters