Capt. Brian Lucas, a military spokesman in Baghdad, said the truck had exploded near the Libyan Embassy in the upscale Mansour district. There were no members of the multinational forces among the casualties.
Rumsfeld, on a Christmas Eve mission to cheer up the troops in Iraq, promised them that no matter how bleak things might look at any one moment they will look back on their mission with pride.
"There's no doubt in my mind, this is achievable," Rumsfeld told troops in Mosul just three days after the devastating attack on a U.S. military dining hall here.
In other recent developments:
Rumsfeld's surprise one-day tour in Iraq took him to the cities of Mosul, Fallujah and Tikrit and the heavily barricaded Green Zone in Baghdad.
He shared a Christmas Eve dinner with troops at a base outside of Baghdad. Wearing a plastic apron, he helped serve food to the troops and later told them as they ate that it would be dangerous to underestimate the power of the insurgency in Iraq.
"I don't want in any way to paint a picture that is pretty," he said.
Rumsfeld said he could not predict whether the level of violence would abate after the Jan. 30 election but said the conflict now amounts to a test of will. "They are determined, but so are we," he said.
At a stop in Tikrit, the hometown of deposed Iraqi President Saddam Hussein, Rumsfeld met with the commander of the 1st Infantry Division, Maj. Gen. John Batiste.
Batiste said that 90 percent of the threat in his area, which covers four provinces in northcentral and northeastern Iraq is from former Baathist regime elements.
He told Rumsfeld the groundwork is being laid for successful elections in this part of Iraq, which is predominantly Sunni.
Standing under a bright sun on a brisk morning, Rumsfeld addressed a group of about 250 soldiers gathered outside Batiste's headquarters, thanking them for their service and wishing them holiday greetings.
His third stop was in Fallujah, where Marine infantrymen fought with insurgents on Thursday as warplanes and tanks bombarded guerrilla positions in the heaviest fighting there in weeks.
However, all seemed quiet as he met with Marines at their main base outside the city on Friday.
"What's taking place here is at a stage where a great many people doubt whether or not it can be accomplished," he said, reprising the theme he struck in Mosul.
But he reminded about 200 Marines eating lunch in a mess hall adorned with several small plastic trees and homemade Christmas decorations that repressive regimes in Germany and the former Soviet Union had been removed and said he was confident freedom would prevail in Iraq.
"All I can say is, people basically want to be free," he said to cheers and applause in the refurbished brick and plaster Iraqi building.
Indeed, the 72-year-old defense chief was greeted warmly by the troops at each stop. He spent a lot of time shaking hands and posing with a number of soldiers who wanted to have their picture taken with him.
He also took questions from the crowds at each stop, but none had the critical edge that he ran into in Kuwait on his most recent visit to the region.
Out of concern for security, Rumsfeld's aides went to unusual lengths to keep his visit a secret prior to his arrival, with only a few reporters and one TV crew accompanying him on an overnight flight from Washington.
In an interview aboard the C-17 cargo plane that brought him to Mosul, Rumsfeld said he'd been planning to visit U.S. troops here long before Tuesday's deadly attack, believed to have been carried out by a suicide bomber.
The blast Tuesday at Forward Operating Base Marez was the deadliest single attack on a U.S. base in Iraq, striking as hundreds of soldiers sat down to lunch. Fourteen U.S. service members were among the 22 killed.
The episode has focused new attention on the ability of the U.S. military to protect its forces.
Rumsfeld's stealth Christmas Eve trip came on the heels of several difficult weeks for the defense chief. Several high-profile Republicans have publicly criticized Rumsfeld, prompting President Bush to defend him Monday as a "good human being who cares deeply about the military and deeply about the grief that war causes."
Rumsfeld has made several visits to troops in the region, most recently two weeks ago to a forward base in Kuwait. There, a handful of soldiers openly challenged him about inadequate equipment and long deployments.