BAGHDAD — Under siege by government forces, al-Qaida militants tried to strengthen their hold on two of Iraq’s main Sunni cities Friday, telling residents that they were defending them against the Shiite-led government.

Iraqi troops and allied tribesmen surrounded the western city of Fallujah and fired artillery at a nearby area where up to 150 militants were stationed, a military official said, speaking on condition of anonymity as he was not authorized to release information. Troops also made forays into the city of Ramadi, he added.