Iraqi Prime Minister Iyad Allawi has admitted for the first time that violence will prevent some parts of Iraq voting in this month's election.
Officers saw their colleagues killed in Tikrit
"There are some pockets that will not participate in the election, but they are not large," he said.
He spoke on a day when at least 15 people were killed across the country.
At least six police officers died in Tikrit, seven Iraqis were killed in an attack south of Baghdad, and at least two died in a bomb in Samarra.
Iraq's interim government has announced it has set aside $2.2bn of this year's budget to strengthen the security forces, who will be responsible for maintaining order on polling day, 30 January.
Mr Allawi said it would fund an increase in the number of Iraqi troops from about 100,000 to 150,000.
"We need to equip the police and army with the new modern weaponry that will enable them to protect the country," he added.
INSURGENT VIOLENCE MOUNTS
11 Jan: 15 Iraqis killed in separate attacks across the country
10 Jan: Baghdad deputy police chief and son shot dead
7 Jan: Seven US soldiers killed in Baghdad bomb attack
6 Jan: Bodies of 18 Iraqis contracted to work at US base found outside Mosul
5 Jan: At least 25 Iraqis killed in three attacks in central Iraq
4 Jan: Governor of Baghdad, 14 Iraqis and five US soldiers killed in separate attacks
3 Jan: More than 20 people killed across Iraq
2 Jan: At least 23 Iraqi soldiers killed by a car bomb in Balad
The blast in Tikrit happened in the north of the town at about 0930 (0630 GMT), the US military said. A dozen people were wounded, police said.
The city, Saddam Hussein's home town - 165km (100 miles) north-west of Baghdad - is one of the centres of the Sunni insurgency in Iraq.
Seven people died in Yussifiya, 15km (9 miles) south of Baghdad.
According to one account, a roadside bomb missed a passing US military convoy and hit a passing minibus instead.
Another report said gunmen opened fire on the vehicle.
Two Iraqis were killed in an attack on a joint US-Iraqi patrol in the city of Samarra, about 95km (60 miles) north of Baghdad.
In other developments:
- The United Nations refugee agency says only about 8,500 of 85,000 residents who have returned to the city of Falluja since a US assault last year, have chosen to stay in their homes
- About 300 lorry drivers - mostly Syrians - are being detained by US forces in Iraq near the border with Syria. The US has made no comment, but has said in the past that Syria is not doing enough to provide security on its border with Iraq.