Over the past month, alleged attacks have targeted four training camps and arms depots used by the Hashed, which is dominated by pro-Iran Shiite groups and is largely opposed to the US.

There have been no claims of responsibility or media access to the facilities, and rumours have swirled of US or even Israeli involvement in targeting the Hashed.

On Wednesday, the paramilitary force said in a statement it had carried out its own investigation and pointed the finger at the US military, but also accused Israel of infringing Iraqi airspace.

"We announce that the first and last entity responsible for what happened are American forces, and we will hold them responsible for whatever happens from today onwards," said the statement, attributed to Hashed deputy chief Abu Mehdi al-Muhandis.

He said the attacks were carried out by "agents or in special operations with modern air planes," without providing further details.

Tensions between Iran and the US have soared since May 2018, when President Donald Trump's administration unilaterally pulled out of a landmark 2015 nuclear accord agreed by major world powers with Tehran.

- 'Israeli drones' -

But Muhandis also said "we have accurate and confirmed information that the US this year allowed four Israeli drones to enter via Azerbaijan... to target Iraqi military headquarters".

He stopped short of explicitly accusing Israel of carrying out raids and also did not make clear whether alleged Israeli drone activity had any relation to the attacks over the last month.

The statement also said Hashed would deal with any foreign planes flying above its positions without the Iraqi government's knowledge as "hostile aircraft."

It comes after at least four suspicious incidents at its positions, starting at a base in Iraq's central Amerli region.

One Iraqi fighter was killed and two Iranians wounded in what the Hashed said was a fire caused by a technical error, but the Iraqi joint operations command attributed it to shelling by "an unidentified drone."

Explosions were then reported at a Hashed base in Diyala, and last week massive blasts went off at the Saqr military camp south of Baghdad.

In reaction, Iraqi Prime Minister Adel Abdel Mahdi ordered an investigation and said any air operations by "Iraqi and non-Iraqi entities" would need his approval.

On Wednesday, a government source who saw the probe's preliminary results said it recorded three warplanes flying near the Saqr base but no proof of shelling.

The final incident took place Tuesday at a Hashed-run position near the Al-Balad airbase, where US troops are present.

The US-led coalition in Iraq declined to respond directly to the Hashed's accusations on Wednesday, referring AFP to the Iraqi government.

Earlier this summer, a string of rocket attacks targeted several bases where US troops are stationed as well as American commercial interests.

- Clipping Iran's wings? -

But observers say the latest reported attacks on the Hashed indicate another player, Israel, may be involved.

Israel has repeatedly warned it would act to stop what it says is Tehran's expansionism in the region, and it has carried out strikes against Iranian forces in Syria.

Israel has not claimed responsibility for the Iraqi incidents but Prime Minister Benjamin Netanyahu hinted at involvement this week, saying his country would "act against (Iran) whenever necessary."

"I've said that Iran doesn't have immunity anywhere, and I meant that," he told journalists in Kiev, after he was asked about the attacks in Iraq.

One of Israel's biggest security concerns is the possibility that Iran could transfer rockets by land to its allies in Iraq, Syria and Lebanon that could then be used to attack Israel.

Security analyst Fadel Abu Raghif said that particular fear may be behind the recent incidents in Iraq.

"We received information about missiles that Iran gave to the Hashed during the (Iraqi) state's fightback against" the Islamic State jihadist group, he told AFP.

"It seems some of those rockets were not used, and that Israel felt the danger," he added.

The Hashed fought alongside the Iraqi military to oust IS from swathes of Iraqi territory the jihadists seized in 2014.

Karim Bitar, a senior fellow at the Paris-based Institute for International and Strategic Affairs, also said Israel was a likely culprit.

"Few people would be surprised if Israel has decided to target Iranian targets in Iraq, as it has openly acknowledged that it has been doing that in Syria," he told AFP.

"This strategy is probably coordinated with the United States, as part of the Trump administration's 'maximum pressure' campaign against Iran," Bitar added.