Britain and France raised the possibility of military intervention in the Syria conflict on Thursday, with the British prime minister joining President Obama in warning that he would not tolerate the transport or deployment of Syria’s chemical weapons stockpile, and France’s defense minister saying that a partial no-fly zone should be considered.

The British and French statements reinforced a sense that the window for diplomatic efforts to halt the 18-month-old conflict might have closed, as new crackdowns by the Syrian military were reported in the Damascus suburbs.

Concern was also growing over the safety of journalists who have entered Syria without official permission to report on the conflict. A veteran Japanese war correspondent was killed on Monday in the embattled northern city of Aleppo, and an American freelance journalist who has been roving Syria with insurgents since May has been missing for more than a week.