The United States military tried, but failed, to kill another senior Iranian commander on the same day a US drone strike killed the Islamic Revolutionary Guard's top general, Qassem Soleimani, US officials said Friday.

The officials said a military air attack targeted Abdul Reza Shahlai, a high-ranking commander in Iran's Islamic Revolutionary Guard Corps (IRGC), but the mission was not successful. The officials spoke to the Associated Press news agency on condition of anonymity in order to discuss a classified mission.

The Pentagon declined to discuss the highly classified operation.

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"We have seen the report of a January 2 air strike in Yemen, which is long understood as a safe space for terrorists and other adversaries to the United States. The Department of Defense does not discuss alleged operations in the region," said Navy Commander Rebecca Rebarich, a Pentagon spokesperson.

The Washington Post first reported the development.

The US Department of the Treasury said Shahlai was based in Yemen and accused him of "a long history of targeting Americans and US allies globally", including killing US-led coalition forces in Iraq. It has offered a $15m reward in connection with him under its "Rewards for Justice" programme.

The US has accused Shahlai of coordinating a failed 2011 plot to assassinate Saudi Arabia's ambassador to the US at a cafe in an upscale neighbourhood of the US capital.

"Shahlai approved financial allotments ... to help recruit other individuals for the plot, approving $5m as payment for all of the operations discussed," the Treasury Department said.

A US drone strike on January 3 killed Soleimani shortly after he landed at Baghdad International Airport. Trump administration officials have justified the killing as an act of self-defence, saying Soleimani was planning military acts that threatened large numbers of US military and diplomatic officials in the Middle East. Iran, however, called the attack an act of "terrorism", and on Wednesday launched more than a dozen ballistic missiles at two bases in Iraq that house US and coalition forces. No one was killed in that retaliation.

The unsuccessful operation against Shahlai may indicate that the Trump administration's killing of Soleimani last week was part of a broader operation than previously explained, raising questions about whether the mission was designed to cripple the leadership of the IRGC or solely to prevent an imminent attack on Americans as originally stated.

The Trump administration has been under fire by Democrats and two Republicans in Congress over its decision to order the drone strike that killed Soleimani.

In an interview that aired on Fox News on Thursday, US Secretary of State Mike Pompeo acknowledged that the US did not know "precisely" when or where the imminent attacks allegedly being planned by Soleimani would take place, but said the threat was "real".

Trump on Friday said Iran probably had targeted the US embassy in Baghdad and was aiming to attack four US embassies.

"We will tell you probably it was going to be the embassy in Baghdad," Trump said in a clip of an interview on Fox News. "I can reveal that I believe it would have been four embassies."

US military operations in Yemen, where a civil war has created the world's worst humanitarian crisis, are shrouded in secrecy. US officials said the operation against Shahlai remains highly classified, and many declined to offer details other than to say it was not successful.