Three Britons and an American are among more than 20 people killed in a day of violence across Iraq.
Monday saw a number of car bomb attacks across the country
The four were in a vehicle hit by a suicide car bomb in Baghdad, one of four such explosions in various parts of the country.
Earlier, a suicide car bomb targeted the offices of interim PM Iyad Allawi, killing four people.
Correspondents say insurgent violence is approaching a crescendo ahead of elections planned for 30 January.
Meanwhile, Iraq's intelligence chief has said more than 200,000 people are taking part in attacks on US and their Iraqi allies.
In other violence on Monday:
- Six Iraqi soldiers were killed by roadside explosions in Tikrit, the hometown of former leader Saddam Hussein
Four Iraqi soldiers were killed when a car bomb struck a checkpoint outside a US military base in Balad
- Two Iraqi security officers were gunned down at a checkpoint in the town of Baiji
- A policeman died in the northern city of Mosul when he tried to remove a booby-trapped beheaded body which then exploded
The BBC's Jim Muir, in Baghdad, says the vehicle carrying the three Britons and one American was caught in a suicide bomb blast at a checkpoint just on the edge of the green zone, the heavily protected international area in the heart of Baghdad.
The identities of the four victims are being withheld while their families are contacted.
The vehicle was owned by an American-based company of security and risk consultants, but not all of those in it were from the same company.
Monday's explosion near the offices of Mr Allawi's party, the Iraqi National Accord, occurred when a car bomber tried to ram through a police checkpoint on the road leading to the building.
Several police vehicles were destroyed in the blast and at least 20 people were wounded.
The National Accord's offices, located inside the heavily fortified Green Zone several hundred metres away, were not damaged.
Mr Allawi is not believed to have been in the building at the time.
Iraqi Intelligence chief Gen Muhammad Abdullah Shahwani told AFP news agency "the resistance is bigger than the US military in Iraq".
He estimated that 40,000 of the 200,000 were core fighters, while the remainder were volunteers and part-timers.
Meanwhile, Iraqi Defence Minister Hazem al-Shaalan has said Iraq's elections could be delayed if the Sunni Muslim community in Iraq agreed to take part, the AFP news agency reported.
"We have asked our Arab brothers, particularly in Egypt and Gulf countries, to get Iraqi Sunnis to participate in the elections and if such participation requires a delay to the election date, they could be delayed," he told AFP.
Iraq's main Sunni political grouping, the Iraqi Islamic Party, has already called for a boycott of the election, and Sunni militant groups have threatened to attack voters.
Significant participation in the election by Iraq's Sunni minority is widely seen as essential to the credibility of the vote.
On Sunday, at least 23 Iraqi soldiers were killed when a car bomb struck the bus they were travelling in Balad, a town in central Iraq's restive Sunni Muslim heartland.