An influential group of lawmakers says launching British airstrikes against Islamic State group militants in Syria would be ineffective without a plan to end the country's civil war.

The Foreign Affairs Select Committee has dealt a blow to Prime Minister David Cameron's attempts to expand British military action against the militants from Iraq into Syria.

Committee chairman Crispin Blunt, a legislator from Cameron's Conservative Party, said Tuesday he feared the government was "responding to the powerful sense that something must be done ... without any expectation that its action will be militarily decisive, and without a coherent and long-term plan for defeating (IS) and ending the civil war."

Cameron and Defense Minister Michael Fallon have said they favor expanding the strikes from Iraq to Syria, but only with Parliament's approval.