 Witnesses said police units sealed off the street outside the ministry |
They included four bodyguards and a finance expert. Earlier reports said the expert was German.
Witnesses and sources told the BBC that the kidnappers wore police uniforms and arrived in up to 40 police vehicles.
"We are aware of reports that a group of Westerners have been kidnapped. We are urgently looking into them," the UK foreign office said.
The foreign office is set to hold an emergency meeting on the situation and is assembling a crisis team.
Also on Tuesday, Baghdad was shaken by a bus explosion which killed at least 22 people and injured about 55, and a car bomb which killed at least 15 people and hurt at least 36.
The US military also announced that two of its soldiers were killed in a helicopter crash on Monday in Diyala province.
Renegade police?
There are conflicting reports about exactly how Tuesday's abduction took place and how many people are missing.
Witnesses said it was carried out by what appeared to be a police unit.
The street was sealed off at both ends and the kidnappers, in police camouflage uniforms, walked straight past guards at the finance ministry building on Palestine Street, the witnesses said.
A police source told the BBC that dozens of police vehicles were used in the operation.
The BBC's Paul Wood in Baghdad says that if such reports are true, it could point to the involvement of a renegade police unit, possibly special commandoes.
While it has been possible in the past for criminals or militants to hire police uniforms and vehicles, he says, the scale of this operation suggests real police involvement.
It is well known that the Iraqi police are heavily infiltrated by militia groups, leading to split loyalties and corruption, our correspondent says.
Fewer kidnappings
The missing security guards are all believed to have been working for the GardaWorld security agency - a Canadian-owned firm largely staffed by British former service personnel.
Their abducted client was thought to be advising the Iraqi finance ministry.
The BBC's security correspondent, Frank Gardner, says a British crisis team, including police hostage negotiators, members of the secret intelligence service, and regional experts, is being assembled to establish lines of communication with the kidnappers.
About 200 foreigners of many different nationalities have been kidnapped in Iraq over the past four years, though the number has fallen dramatically since a few years ago.
This is thought to be the first time an abduction has been staged at a government facility.
Correspondents say hopes for the captives' future depends on who is holding them.
The Sunni al-Qaeda has a record of killing captives, while if it is a Shia group, there may be scope for political negotiation, the BBC's Paul Wood in Baghdad says.
If it is a criminal group, then GardaWorld can expect a ransom demand, he says. However, the British government has a declared position of not paying ransoms.
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