BAGHDAD — A car bomb exploded in a largely Shiite neighborhood of Baghdad today and killed at least 16 people, police said, while Defense Secretary Gates said during a visit to the capital that security and stability were within reach, although more work is needed.

It was the fourth deadly car bomb in Iraq today. Earlier, a blast went off in the northern city of Mosul, where Mr. Gates had landed on his sixth trip to Iraq.

Gunfire and sirens followed the bombing in Baghdad’s Karradah neighborhood, and a plume of smoke rose in the sky.

Police and hospital officials said at least 16 people were killed and 32 wounded in the explosion, which took place across the Tigris River from the Green Zone shortly before Mr. Gates’ news conference with Defense Minister Abdul-Qader al-Obeidi.

“I believe that a secure, stable Iraq is within reach,” Mr. Gates said, adding however: “We need to be patient.”

He said that Iraqis who have been fighting Al Qaeda and other military forces on the local level must be integrated into the formal Iraqi security forces.

Mr. Gates noted a decline in overall violence in Iraq during recent months, that has led to “a growing sense of normalcy and hope.” But he acknowledged increasing militant and terrorist activities in northern Iraq.

A brigade commander, Army Colonel Tony Thomas, said senior commanders in the north are looking for additional American troops and also would like the return of 1,400 Iraqi troops sent to Baghdad to provide “more combat power” to help stabilize areas such as Diyala province, Mosul, and Samara.

America is pressing Iraqi leaders to take advantage of the improved security to make the political reforms needed to stabilize the fledgling democracy.

The American military also announced the deaths of three soldiers from a “complex attack” yesterday involving a roadside bomb and small arms fire north of Baghdad, the American military said. A fourth soldier was wounded, it said.