Three explosions in the Sunni-dominated city of Fallujah on Monday morning killed seven people in the span of about 15 minutes.

The first came at 9:45 a.m., when a booby-trapped car exploded on an Iraqi army patrol, killing one officer and three soldiers, according to Lt. Ali Aldullamy of the provincial police. As security forces arrived on the scene 10 minutes later, two explosive charges went off, killing two police officers and one civilian, Aldullamy said.

More than 20 civilians, soldiers and police officers were injured in the attack, which officials blamed on al-Qaeda-linked insurgents.

Though security has improved dramatically across Iraq, it remains fragile, and many Iraqis are worried the situation will deteriorate if U.S. forces withdraw as scheduled in December. In particular, al-Qaeda-linked militants have been blamed in some of the worst attacks in recent months.

A highly orchestrated Feb. 26 bombing temporarily shut down Iraq’s biggest oil refinery, a rare assault on such a facility, as opposed to the more vulnerable pipelines. And a March 29 raid on a provincial headquarters in Tikrit, which the insurgents claimed was carried out by two gunmen, killed more than 50 people.

Although a degree of regular life has returned to Baghdad and other areas, the atmosphere in Fallujah and other Sunni-dominated cities — places once considered insurgent strongholds — remains tense.

On a trip here last week, U.S. Defense Secretary Robert Gates raised the possibility of troops staying longer if the Iraqi government asks, a politically fraught matter for Prime Minister Nouri al-Maliki.

His fragile governing coalition includes the populist Shiite cleric Moqtada al-Sadr, who issued a statement over the weekend vowing to return his Mahdi Army militia to the streets if the Americans stay.

As they do every year, tens of thousands of Sadr supporters protested in the streets Saturday, burning U.S. flags to mark the symbolic anniversary of the fall of Baghdad and U.S. invasion.

Special correspondents Othman Almukhtar in Fallujah and Aziz Alwan in Baghdad contributed to this report.