Baghdad ? Anti-American Shiite militia leader Muqtada al-Sadr ordered his followers Saturday to defy government orders to surrender their weapons, as U.S. jets struck Shiite extremists near Basra to bolster a faltering Iraqi offensive against gunmen in the city.

Prime Minister Nouri al-Maliki acknowledged he may have miscalculated by failing to foresee the strong backlash that his offensive, which began Tuesday, provoked in areas of Baghdad and other cities where Shiite militias wield power.

Government television said the round-the-clock curfew imposed two days ago on the capital and due to expire today would be extended indefinitely.

The U.S. Embassy tightened its security measures, ordering all staff to use armored vehicles for all travel in the Green Zone and to sleep in reinforced buildings until further notice after six days of rocket and mortar attacks which left two Americans dead.

Despite the mounting crisis, al-Maliki, himself a Shiite, vowed to remain in Basra until government forces wrest control from militias, including al-Sadr’s Mahdi Army. He called the fight for control of Basra “a decisive and final battle.”

British ground troops, who controlled the city until handing it over to the Iraqis last December, also joined the battle for Basra, firing artillery Saturday for the first time in support of Iraqi forces.

Iraqi authorities have given Basra extremists until April 8 to surrender heavy and medium weapons after an initial 72-hour ultimatum to hand them over was widely ignored.