Women and children were among the casualties of the 10:00 am (0700 GMT) attack near a train station in the al-Maakal district of north Basra, Ghanimi said, but it was unclear how many of them died or were wounded.
Sergeant Sarah Snedden, a spokeswoman for US forces in Iraq, confirmed there had been "an incident in Basra" but did not provide further details. Violence has declined dramatically across Iraq since its peak in 2006 and 2007, but attacks remain common, with 197 people dying as a result of violence in February.
The presence of US forces in Iraq's streets is also rare the 50,000-odd troops stationed in the country are mainly tasked with training and equipping their Iraqi counterparts, though they do still conduct joint counter-terror operations. All US forces must withdraw from Iraq by the end of the year, under the terms of a bilateral security agreement.