The US military says it is stepping up security in Baghdad, after twin bombs rocked the heavily-fortified Green Zone of the Iraqi capital.
The Green Zone is not as safe as once thought
Up to six people died in the attack, the first to occur in what is supposed to be the safest place in Baghdad.
The blasts took place within seconds of each other - one in a cafe, the other in a street lined with souvenir stalls.
Supporters of Abu Musab al-Zarqawi - who the US says has links with al-Qaeda - have claimed the bombings.
In a statement posted on an Islamic website, the group led by the Jordanian militant described the attack as one of their "most successful operations".
A US military statement said security would be "significantly increased for an undetermined period" in the Green Zone and at other key locations in and around Baghdad.
The new measures include more armed patrols, intensified security at Baghdad airport, and air patrols.
The Green Zone, where the US and UK embassies are also located, is often targeted by mortars.
There have been suicide bombings at its entrances - but this was the first time an attack had been carried out from within.
The first blast targeted a cafe popular with both soldiers and civilians.
"People were screaming... People were stampeding, trying to get out," one eyewitness told AFP news agency.
The cafe was gutted by the blast
Seconds later, the second bomb rocked a nearby bazaar.
The US State Department says the bombs killed three US contractors, and a fourth is missing, presumed dead.
Two Iraqis died and about 20 people were injured, it added.
"This cowardly act will not go unpunished," said Iraq's national security adviser, Kasim Daoud.
Initial reports suggested the attacks were a "suicide operation", he added.
The Green Zone, set up last year, is supposed to be the one area of Baghdad that is safe to foreign contractors.
However a bomb was defused outside a popular restaurant in the zone earlier this month, prompting the UK embassy to advise staff to avoid restaurants.