The violence was the latest in a string of deadly attacks targeting Iraqi forces and others allied with the U.S. military that have killed at least 68 Iraqis since Friday. The surge in bloodshed has come despite major U.S. offensives last month to suppress guerrillas ahead of elections set for Jan. 30.
In other developments:
The gunmen in Tikrit opened fire from two cars at the bus as it dropped off Iraqis employed by coalition forces, said Capt. Bill Coppernoll, spokesman for the U.S. 1st Infantry Division.
Coppernoll said 17 people died and 13 wounded in the attack, which occurred at about 8:30 a.m.
Survivors reported that the gunmen emptied their clips with a spray of gunfire into the bus, then fled, Coppernoll said. The survivors said about seven guerrillas were involved in the attack.
About an hour later, a suicide car bombers drove into an Iraqi National Guard checkpoint in Beiji, about 75 miles to the north, detonating his explosives-packed vehicle, Coppernoll said. Then gunmen opened fire on the position.
Three guardsmen were killed and 18 wounded, Coppernoll said.
Guerrillas also attacked patrolling guardsmen near Samarra, 60 miles north of Baghdad, early Sunday, killing one and wounding four.
Insurgents have routinely targeted Iraqis employed by the U.S.-led coalition and Iraqi security forces, accusing them of collaborating with the U.S.-led occupation forces. The attacks have also taken on a new impetus in the run-up to Jan. 30 elections as insurgents attempt to derail the ballot.
Insurgents also pursued their deadly campaign against American troops and Iraqi security forces. Two U.S. soldiers were killed by roadside bombs in Baghdad and near Baqouba Saturday, and two other American soldiers with Task Force Olympia were killed and four wounded when their patrol came under attack in the turbulent northern city of Mosul.
The two soldiers were killed during a patrol in Mosul's Palestine neighborhood Saturday, when they came under fire from insurgents shooting from two mosques and other buildings in the area, according to spokeswoman Capt. Angela Bowman. The U.S. military later returned to the area and detained three suspects.
The killings brought to at least 1,271 the number of U.S. troops to have died since the war began in March 2003, according to an Associated Press count.
The U.S. military also announced that 64 suspected insurgents had been arrested on Saturday in Avgani, a town west of Mosul, a city which has been the scene of increased fighting as insurgents.
A suicide car bomber in Mosul killed seven Kurdish militiamen Saturday belonging to the Patriotic Union of Kurdistan, one of Iraq's two largest Kurdish groups. Two bystanders also died.
In nearby Hawija, gunmen assassinated an official of the Kurdish party, Jalal Dawood, after knocking at the door of his house and shooting him in a hail of gunfire.
Along with Iraq's majority Shiites, Kurdish political parties back the upcoming elections.
Mortars rounds fired toward a U.S. military base in Ramadi, west of Baghdad, missed their mark Sunday and landed on a residential area, killing an Iraqi woman and a child, hospital officials said.
In nearby Habinyah, 1,500 Iraqis displaced by recent fighting in the insurgent stronghold of Fallujah demanded the government allow them to return them home.
Meanwhile, the militant Jaish Muhammad, Arabic for Muhammad Army, issued a statement saying its fighters were lying low for "a few days" ahead of an imminent resumption of attacks against U.S. forces.
The group's statement, which could not be immediately verified, also threatened Iraqis against aiding coalition forces that they would be attacked with similar fury as that directed against the U.S. military.
Officials had hoped the Fallujah assault would put the rebels on the defensive throughout Iraq. But the latest attacks, including a Baghdad suicide bombing at a police station that killed seven Saturday, showed they remain capable of hitting where they choose.