One round fell into the embassy's compound in the heavily fortified Green Zone in central Baghdad on Saturday, according to a US embassy official.
Embassy spokesman Bob Callahan confirmed the embassy had been hit in an attack, but could give no details about casualties.
The second official confirmed that two had been killed and four injured.
Earlier in the day, 17 people were killed in a spate of attacks across Iraq.
As security forces barricaded streets, sealed Iraq's borders and closed Baghdad airport, more than a dozen polling stations were attacked and bloodshed continued to stain the electoral countdown.
Interim Prime Minister Iyad Allawi appealed to Iraqis to defy violence "trying to break us and to break our world" and exhorted voters to cast ballots in Iraq's first multi-party election for half a century.
Many candidates have kept their
names secret due to safety fears
With just a day to go before the polls, a bomber blew himself up outside a US-Iraqi security centre in the town of Khanaqin, northeast of Baghdad near the border with Iran.
The US military said three Iraqi soldiers and five civilians were killed. No Americans were hurt.
Most other attacks were concentrated north of Baghdad where fighting has been fiercest and where many Sunnis plan to boycott the election.
Three civilians were killed in a roadside bombing in the city of Samarra, and a rocket attack on an Iraqi army base in the town of Duluiya killed three soldiers.
Fighters blew up an explosives-laden donkey cart outside a polling station, killing a guard, in the town of Sharqat, south of Mosul, witnesses said. Mortar rounds hit a voting centre in the refinery town of Baiji, wounding four guards, after two other sites were dynamited there overnight.
Three Iraqi contractors abducted a week ago were found shot dead in the town of Balad. Fighters brand all Iraqis working with US forces as collaborators and have killed hundreds.
The election forms the cornerstone of the Bush administration's plan to transform Iraq from dictatorship to democracy after the fall of Saddam Hussein in April 2003. But it risks fuelling the violence and fomenting sectarian strife.
With tension high in the capital, bursts of gunfire echoed in some areas and fighters fired rocket-propelled grenades at the fortified Baghdad Hotel. There was no word of casualties.
Allawi called on Iraqis to defy
violence and vote
South of Baghdad, an Iraqi woman and her child were killed when mortar rounds targeting a US base in Musayyib struck their home, security officials said.
US troops killed two Iraqis in a car near the western city of Ramadi, according to witnesses. The circumstances were not immediately known.
The climate of intimidation has been so prevalent that most candidates have kept their names secret.
There are fears that the turnout will be lowest in Sunni Arab areas, where violence has been bloodiest. Sunnis make up 20% of the population.
Iraq's Shia population, is expected to dominate the polls, bringing them to power for the first time in the country's history.
Low Sunni representation in the new 275-member National Assembly could undermine the government's credibility.
Many people vowed to brave the threats, but others were afraid of being targeted at the polls or afterwards, when indelible blue ink daubed on their index fingers to prevent multiple voting could mark them for death.
A survey by Zogby International showed 76% of Sunni
Arabs did not plan to vote. Only 9% said they would
definitely vote. By contrast, 80% of Shia were expected to take part.