LONDON — Britain's Foreign Office says it is looking into reports that a number of British nationals have been killed fighting alongside al-Qaida-linked groups in Syria.

Western officials have long warned that the civil war in Syria is attracting foreign fighters from European countries, many of whom have joined hardline Islamic militant groups.

Earlier this month, the head of Britain's domestic spying agency said the intelligence service had seen "low hundreds of people" from Britain go to Syria and that some have since returned — stoking fears extremists could pose a domestic threat.

On Thursday, the Times of London newspaper reported that four British men have been killed in recent months fighting alongside rebels battling President Bashar Assad's forces.