BAGHDAD, Iraq – In the first significant setback to Baghdad’s five-day old security plan, a twin car bombing killed at least 60 people in a Shiite district of the city Sunday, tempering the cautious optimism that had formed as the strict emergency measures began to take hold.

The two bombs exploded moments apart in a crowded market in the Baghdad al-Jadid neighborhood, shattering the relative calm that had prevailed since the plan went into effect last week with an attack that echoed the wave of bombings at open-air markets in which hundreds of people died in the weeks before the plan was put in place.

Two U.S. soldiers were killed in separate attacks on their patrols in Baghdad, a suicide bomber killed an Iraqi soldier on the edge of the Shiite enclave of Sadr City and a sniper killed three civilians in a central Baghdad neighborhood.

The first days of the much-trumpeted security plan, billed by many as the last hope for a city on the brink, saw a sharp reduction in the levels of violence, prompting Prime Minister Nouri al-Maliki to declare it “an outstanding success” in a telephone conversation with President Bush on Friday.

U.S. commanders have been more cautious, casting the new strategy to secure Baghdad as a long-term endeavor that may not produce results until autumn. Most of the promised surge of 17,500 U.S. forces due to deploy in Baghdad will arrive in May, though the Pentagon announced Saturday that it was moving to March the dispatch of the 3rd Infantry Division’s headquarters, previously scheduled for June. The unit will provide support for the plan, which will see the number of U.S. troops in Baghdad surpass 25,000 for the first time since the 2003 invasion.

U.S. ambassador Zalmay Khalilzad urged patience in a statement condemning the bombings. “Heinous acts such as these will serve only to galvanize Iraqi forces and their coalition partners who are continuing to move forward in securing Baghdad,” he said. “The brave Iraqi people know that this will require resolve. They know that it will not happen overnight.”

Much is riding on the success of this effort to bring Baghdad under control, including America’s hopes for an eventual withdrawal of U.S. forces and Iraqi hopes that their country can avert an all-out civil war.

Already, the plan has had a tangible effect on city life, with a greater number of patrols and checkpoints in many parts of the city. Convoys of U.S. Army Humvees, a relatively unusual sight in past days, now weave their way through traffic-clogged streets, as crisply uniformed Iraqi army soldiers check IDs and search cars for weapons. B-1 bombers and F-16 fighter jets circle regularly overhead, in a noisy reminder of the firepower behind this latest push.

Most significantly, police have reported a sharp decrease in the number of corpses dumped around the city, with only five bodies found Sunday, compared to a typical daily body count of about 50.

The reduction is seen as a sign that the execution-style killings considered a hallmark of the Shiite militias have tapered off.

Indeed, one distinctive feature of the plan has been the noticeable absence of the Shiite Mahdi Army militia members from the Baghdad neighborhoods that had been under its control, most of them in the eastern sector of the city.

Bush has insisted that this plan, unlike the last one, will target Shiite militias as well as the Sunni insurgents held responsible for the majority of bombings in the capital, such as Sunday’s market attack.