Insurgents have killed 21 Iraqis in a series of attacks across the country.
Insurgents are targeting Iraqis more and more
Seventeen civilians working for the American military died and 13 more were hurt when gunmen opened fire on a bus taking them to a base in Tikrit.
Elsewhere on Sunday, four members of the Iraq security forces were killed in the towns of Beiji and Samarra.
Continuing violence has raised concerns over January's planned election but interim Foreign Minister Hoshyar Zebari says the poll must and will be held.
"The elections are difficult but they are do-able in my view," Mr Zebari told the BBC World Service.
He said the government was not underestimating the security threats but was striving to do everything to make sure they are held by 30 January.
Postponing the election would delay subsequent steps in Iraq's political transition, he said on the Newshour programme.
"So this deadline will drag on and on and you will hand those terrorists, those extremists, a victory."
The top US commander in Iraq, Gen John Abizaid, has said that local forces are not up to the task of protecting the election by themselves and still need US help.
Sunday's attacks on people working with or for coalition forces took the number of Iraqis killed since a resurgence of violence on Friday to at least 68.
Capt Bill Coppernoll, of the US 1st Infantry Division, said gunmen opened fire as Iraqis arrived for work at a base in Tikrit, Saddam Hussein's hometown, at about 0830 (0530 GMT).
"Two vehicles opened fire on a civilian bus that had stopped to let the workers off, who were employed by coalition forces," he said.
Capt Coppernoll said there was another attack about an hour later when a suicide car bomber drove into an Iraqi National Guard checkpoint in Beiji, about 120km (75 miles) north of Tikrit.
In Samarra, south of Tikrit, one Iraqi soldier was killed and four were wounded when insurgents attacked their patrol with rocket-propelled grenades and small arms fire, he said.
US forces also continue to be targeted by insurgents. The US military said six US soldiers were killed on Saturday.
Money and motivation
Gen Abizaid, the head of US Central Command, called on Iraq's neighbours - in particular Syria and Iran - to do more to curb the flow of foreign fighters into Iraq.
He told a regional conference on Gulf security in Bahrain that Iraqi troops did not have the training or experience to do the job without continuing US help.
"It's very important for everybody to realise that the stability of Iraq is as dependent on its neighbours as it is on the people inside Iraq," he said.
"If the neighbours allow groups of people who are against the stability of Iraq to operate from their territory, then we have a very difficult situation," he added.
He said it was clear that former regime members had the money and motivation to help insurgents "and we have asked the Syrian government to put a stop to that".
But he said he was disappointed that the Iraqi army was still developing too slowly to cope with the security situation.
The Pentagon announced that the US will boost its troop levels to 150,000 by mid-January - an increase of 12,000 personnel.