A Katyusha rocket was fired at a US-Iraqi training base in the city of Mosul late on Tuesday, in the second such attack in almost 24 hours.
About 11.15pm, the rocket was fired at the Presidential Palaces Compound in the east of the city, where US trainees are stationed alongside Iraqi forces, Sky News Arabia reported.
The attack is the second this week, after three Katyusha rockets were fired at the Taji joint US-Iraqi base north of Baghdad.
The attacks come despite US pressure on Baghdad to rein in pro-Iranian proxies and other militias in the country. US Secretary of State Mike Pompeo called Iraqi Prime Minister Adel Abdul Mahdi last Friday and discussed Iranian threats.
Mr Pompeo “appreciated Prime Minister Al Mahdi’s commitment to protect US personnel in Iraq, supporting the Iraqi people and continue the defeat ISIS campaign, and his continued efforts to counter threats to Iraq’s sovereignty from Iran-backed militias", a statement read.
On Wednesday, Kuwaiti Emir Sheikh Sabah is expected to make a rare visit to Baghdad to discuss co-operation and regional tension.
Meanwhile, US President Donald Trump said on Wednesday that his government was "very prepared" for Iran, which he called “a nation of terror".
He said that when his predecessor Barack Obama signed the nuclear deal with world powers and Tehran, Iran’s leaders were screaming 'Death to America'.
"I haven't been hearing that lately,” Mr Trump said.
But last February, Iran’s supreme leader Ayatollah Ali Khamenei insisted that the Iranian people "will not stop saying 'Death to America' as long as the US acts maliciously" towards Tehran.
Nicholas Heras, a senior fellow at the Centre for New American Security, said the US was being challenged by the rising tension in Iraq.
"The US military is being put on notice that it is vulnerable in Iraq and that it best remember that if it chooses to escalate its presence in the region," Mr Heras told The National.
The challenge in Iraq, where the US has about 5,000 troops, is that the attacks "put the burden on the United States to show force to deter its opponents", he said.
Katyushas are neither expensive nor difficult to launch, Mr Heras said.
Many non-state actors in Iraq could have access to the rockets, he said, but Iran-backed groups have an advantage because they they are mostly incorporated into Iraq's security forces.
This means they have the ability to store and launch Katyushas.