Three Iraqi children and two women from the same family were killed Monday when a rocket targeting Baghdad airport, where US troops are stationed, fell instead on their home, the army said.
The latest in a string of attacks targeting American interests in Iraq, it came after Washington threatened to close its embassy and withdraw its 3,000 troops from the country unless the rockets stop.
Attacks over the past year have caused relatively few casualties, and Monday's incident was notable for the number of civilians killed.
The army said it also wounded two children.
Twitter accounts supporting US arch-enemy Iran regularly praise the attacks, but that was not the case Monday, when no group immediately claimed responsibility.
Half a dozen previously unheard-of factions have claimed similar attacks in recent months, under the banner of "Islamic resistance" against the "American occupier".
But experts say they are a smokescreen, and that they include former members of pro-Iranian factions of the Hashed al-Shaabi paramilitary alliance, a state-sponsored network close to Tehran.
The deaths of civilians could put the group responsible in an uncomfortable position with a public exhausted by years of violence by various armed groups.
The Iraqi army on Monday accused "criminal gangs and groups of outlaws" of seeking to "create chaos and terrorise people".
Between October and July, at least 39 rocket attacks targeted US interests in Iraq. Almost the same number again have taken place since.
In total, four soldiers -- two British, one Iraqi and one American -- along with a US and an Iraqi contracter have been killed in the attacks, while several civilians have been wounded.
Iraqi intelligence sources have blamed the attacks on a small group of hardline Iran-backed paramilitary factions.
- 'Honeymoon is over' -
Senior American officers today see pro-Iran armed groups as a greater threat than the Islamic State jihadist group, that once held a third of Iraq.
Washington has demanded that Baghdad take decisive action.
But Iraq must play a delicate balancing act with US influence and that of neighbouring Iran, which arms, finances and supports various armed Shiite factions.
US Secretary of State Mike Pompeo called Iraqi President Barham Saleh this month and threatened to close the American embassy in Baghdad, Iraqi and foreign officials told AFP.
That was seen as a new blow to premier Mustafa al-Kadhemi, who took office as prime minister in May.
While he is seen as western-leaning, US officials have accused him of not doing enough against pro-Iran groups.
One Iraqi official told AFP: "The honeymoon is over."
The American ultimatum was followed by threats of sanctions against senior political and military figures.
Populist Shiite cleric and politician Moqtada Sadr has since called for a commission of inquiry into the rocket attacks, a proposal backed by Kadhemi.
The Hashed paramilitary alliance has dismissed several commanders accused of links to attacks on western interests, while denying responsibility for the acts of groups claiming Hashed links and "carrying out illegal military acts against foreign interests".
But more hardline groups have stepped up the anti-US rhetoric.
One western official told AFP on condition of anonymity: "If Washington follows through and withdraws its people, these groups will be able to brag that they kicked the Americans out of Iraq at little cost.