"The prime minister has not ruled anything out," the spokesman said, adding that Hammond's comments were in reference to a parliamentary vote last year on military action in Syria which the government lost.
But the spokesman added: "No decisions have been been taken".
Speaking during a visit to Berlin earlier today, Hammond said: "Let me be clear, Britain will not be taking part in any air strikes in Syria".
Cameron has previously said he would act against the militants to protect Britain's national interest even without parliamentary approval, although Britain has so far limited to action against IS in Iraq providing humanitarian aid, arms for Kurdish fighters and logistical support for US raids.