The U.S. military had no immediate comment on the report, which came a day after the first American airstrikes were launched in Basra during a week-old offensive against militant followers of radical Shiite cleric Muqtada al-Sadr.
Seven other people were wounded when the plane fired on a house in Basra's Hananiyah neighborhood overnight, a local policeman said on condition of anonymity because of security concerns.
It was not immediately possible to independently verify whether those killed were civilians or combatants.
While the Iraqi police officer claimed it was a U.S. plane, British jets also have been providing air support in the area; it couldn't be immediately confirmed whether the plane was British or American.
The British military had no immediate information but said it also was looking into the reports.
"We are aware of reports of incidents in the Basra area resulting in civilian casualties," said Maj. Tom Holloway, a British military spokesman. "We are investigating those reports and do not have any further details at this time."
AP Television News footage showed smoke rising from Hananiyah. Pools of blood and a destroyed pickup truck were seen outside the home hit by the plane.
American support in Basra came as Iraqi troops struggled against strong resistance in the city, the nation's commercial center and headquarters of the vital oil industry. Clashes there have sparked retaliatory fights in Baghdad and other Shiite cities.
U.S. military intelligence analysis of the fighting in Basra indicated Iraqi security forces controlled less than a quarter of the city, CNN reported on Saturday, citing unnamed officials in the U.S. and Iraq. The analysis also said militia members have deeply infiltrated Basra's police units.
The fight for Basra is crucial for the government of Iraqi Prime Minister Nouri al-Maliki, who is staking his credibility on gaining control of the city, Iraq's second largest, which has essentially been held by armed groups for nearly three years. Al-Maliki flew to Basra earlier this week to personally assume command of the operation and has vowed there would be "no retreat."
The crackdown in Basra has provoked a violent reaction - especially from al-Sadr's Mahdi Army. His followers accuse rival Shiite parties in the government of trying to crush their movement before provincial elections this fall.
Their anger has led to a sharp increase in attacks against American troops in Shiite areas following months of relative calm after al-Sadr declared a unilateral cease-fire last August and recently extended it for six months.
In extracts of an interview broadcast by the Al-Jazeera television network, al-Sadr called Saturday for Arab leaders to voice their support for Iraq's "resistance" to what he calls foreign occupation.
Many Shiite militias, including the Mahdi Army, are believed to receive weapons, money and training from nearby Iran, the world's most populous Shiite nation.
