An alliance of insurgent groups linked to al-Qaeda says it carried out the deadly attack on the Iraqi parliament.
The Islamic State in Iraq said in a statement it had sent a "knight" into Baghdad's highly-fortified Green Zone.
The claim came as lawmakers attended a special session of parliament to condemn the attack, which left one lawmaker dead.
In New York, the UN Security Council also condemned the bombing, describing it as a "heinous act of terrorism".
In a statement read by the current chairman, UK envoy Emyr Jones Parry, the council called for "perpetrators, organisers, financiers and sponsors" of acts of terrorism in Iraq to be brought to justice.
The suicide attack on Thursday - on a restaurant used by lawmakers - also left around two dozen people injured.
Initial reports by the US military said eight people had died, but this number was later revised.
The attack has raised questions over how the bomber managed to enter the heavily-guarded zone. It also comes as a blow to the US-led security surge, now in its third month.
The Islamic State in Iraq - which includes a number of Sunni insurgent movements - said it carried out the attacks in a statement posted on an Islamist website.
"A knight from the state of Islam... reached the heart of the Green Zone" and exposed the lies about the security surge in Baghdad, the statement said.
It is impossible to say whether the claim is genuine but one institute that monitors militant websites believes that it is, says the BBC's Jonathan Charles in Baghdad.
It is also the sort of headline-grabbing attack carried out by the organisation in the past, our correspondent adds.
One report said police were questioning three workers from the cafeteria where the bomb went off.
The claim came shortly after lawmakers held a rare emergency session of parliament - on what is normally a rest day - to condemn the attack.
The bomb exploded as lawmakers and staff were having lunch
Opening the meeting, Speaker Mahmoud Mashhadani said it was "a clear message to all the terrorists and all those who dare try to stop this (political) process".
"We ought to sacrifice all that is dear for its success and continuation," he said.
The sitting began with readings from the Koran and many tributes to the minister who died, Mohammed Awadh, who was from one of the Sunni factions.
Lawmakers wounded in the attack were also present.
"We have to forget our pain and unite again for the sake of Iraq," said Sunni woman MP Razha Hamdun Abdallah, who wore a bandage on her neck.
Investigations are underway into how the bomber managed to pass through checkpoints to enter one of the most stringently guarded buildings in the country.
This is the first time a bomb has gone off inside the parliament building, although it has been shaken by several mortar attacks in the past.
Also on Thursday, a bomb attack on a key Baghdad bridge, killed at least eight people and sent several cars into the River Tigris.
Correspondents say the attacks come as a blow to the US-led security drive which has brought down the rate of sectarian murders but not an end to the bomb attacks.