The United States is imposing additional sanctions on Iran as a result of its attack on US troops in Iraq this week, Treasury Secretary Steven Mnuchin has said.

"We are announcing additional sanctions against the Iranian regime," Mr Mnuchin said at a White House news conference with Secretary of State Mike Pompeo.

The sanctions will target eight top Iranian officials that the US accused of working to destabilise the region, as well as more than a dozen leading producers of metals in Iran.

Mr Pompeo also said the US believes it is "likely" that a Ukrainian airliner that was shot down over Tehran earlier this week was hit by an Iranian missile.

All 176 people on board the aircraft died in the incident. Many of them were travelling from Tehran to the Ukrainian capital, Kiev, en route to Canada.

Meanwhile the United States has rebuffed an Iraqi request to prepare to pull out its troops, amid heightened US-Iranian tensions following the US killing of an Iranian commander in Baghdad last week.

Iraq looks set to bear the brunt of any further violence between its neighbour Iran and the US. Its leaders are caught in a bind as Iran and US are also the Iraqi government's main allies and vie for influence there.

Iraqi Prime Minister Adel Abdul Mahdi made his request in a phone call with Mr Pompeo late yesterday in line with a vote by Iraq's parliament last week, his office said in a statement.

Mr Abdul Mahdi asked Mr Pompeo to "send delegates to put in place the tools to carry out the parliament's decision", it said, adding without elaborating that the forces used in the killing had entered Iraq or used its airspace without permission.

However, the US State Department said any US delegation would not discuss the withdrawal of US troops as their presence in Iraq was "appropriate."

"There does, however, need to be a conversation between the US and Iraqi governments not just regarding security, but about our financial, economic, and diplomatic partnership," spokeswoman Morgan Ortagus said in a statement.

The latest flare-up in the long shadow-war between Iran and the United States started with the killing of Iran's top general Qassem Soleimani in a US drone strike on 3 January. Iran responded on Wednesday by firing missiles at US forces in Iraq.

In the aftermath, both sides backed off from intensifying the conflict but the region remains tense, with Iranian commanders threatening more attacks. 

Iraq's top Shia Muslim cleric has condemned the confrontation taking place on Iraqi soil, saying it risked plunging an already war-ravaged country and the wider Middle East into deeper conflict.

Grand Ayatollah Ali al-Sistani said it was Iraqis who stood to suffer most from the conflict.

In a message delivered through a representative at Friday prayers in the holy city of Kerbala, Ayatollah Sistani said no foreign powers should be allowed to decide Iraq's fate.