The United States pressed Iraq to explain media reports that it had signed a contract to buy arms from Iran, a move forbidden under a United Nations embargo.
"We've certainly seen those reports. If true, this would raise serious concerns," State Department spokeswoman Jen Psaki said.
A deal signed in November in which Iraq would buy $195 million in arms from Iran was reported by Reuters in Baghdad Monday.
Psaki said that "any transfer of arms from Iran to a third country is in direct violation" of the UN embargo.
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"We are seeking clarification on this matter from the government of Iraq and to ensure that Iraqi officials understand the limits that international law places on arms trade with Iran," she added.
The contract with Iran would have been signed just after Iraqi Prime Minister Nuri al-Maliki visited Washington requesting American military aid in fighting al-Qaeda and other Sunni extremist groups.
In early January, Iran's deputy chief of staff General Mohammad Hejazi said that his country was ready to provide military equipment and advice to Iraq as it battles al-Qaeda. Maliki payed a visit to Iran one month later.
To help Iraq fight al-Qaeda and other Sunni extremist groups, such as the jihadist Islamic State of Iraq and the Levant (ISIL), Washington has recently promised to speed up its delivery of Hellfire missiles and surveillance drones. The Pentagon is ready to sell more missiles and 24 Apache attack helicopters.
"We certainly view the government of Iraq as a partner in the fight against terrorism and we're committed to supporting them in this fight," Psaki said.
She added that the United States has provided more than $15 billion in equipment, services and training to Iraqi armed services and security.