As the clock ticks toward full US military withdrawal from Iraq, American officials who want troops to stay longer continue to warn of a growing Iranian threat.

Some argue that the diminishing US presence is turning Iraq into an even-more contentious regional battlefield, giving rise to a low-grade war between the remaining American forces and what the US says are militias tied directly to Iran’s Revolutionary Guard.

Prime Minister Nouri al-Maliki, whose rule is dependent on Iraqi parties with ties to Iran, appears unable or unwilling to crack down on the most lethal Iranian-back militias, blamed for June attacks that killed the largest number of American forces in two years.

While military officials say Iraqi security forces have continued to fight Shiite militias linked to Iran, the US is increasingly conducting attacks alone – attacks authorized for self-defense under the status of forces agreement between Washington and Baghdad that expires this year.

“We always want to work with the Iraqi security forces … but we’re not going to sit back and get shot at and can’t defend ourselves. So if we can’t have the help or don’t have the help then we will in fact act to defend ourselves,” says US military spokesman Maj. Gen. Jeffrey Buchanan.

For example, says General Buchanan, US Apache helicopters fired on Iraqis shooting rockets at the US base at the Basra airport last month.

“They engaged and killed them – that was an act of self defense. It would have been far better for all had the Iraqi Army been able to prevent the attack or respond to it and stop it in action but things like that happen... what we’re not going to do is sit back and watch them shoot at us and wait for the Iraqi army to show up,” he says.

15 American fatalities in June

Rocket, mortar, and roadside bomb attacks in June killed 14 US servicemen and an American civilian contractor. Buchanan says all but two of the fatal attacks were conducted by three major Shiite militias with ties to Iran.

Iraq's Foreign Minister Hoshyar Zebari on Wednesday said it would be difficult for a new agreement with the US to pass through parliament. He said Iraq was likely to instead sign more limited agreements on the Defense Ministry level for American trainers and advisers to remain in Iraq.

In the oil-rich south – seemingly one of the calmest areas of the country – recent attacks appear to have taken some of the US military by surprise. Some soldiers live in trailers with little of the protection from mortars and rockets that is common in the rest of the country.

US Gen. Martin Dempsey, in Senate confirmation hearings Tuesday, said the heightened militia campaign could be the run-up to a huge attack similar to the 1983 bombing of Marine barracks in Beirut that drove US forces out of Lebanon.

Analysts say the attacks follow an Iranian strategy of trying to exert security, political, and economic influence in Iraq that the US has found difficult to counter.

“The Iranians are good at this and they are continuing to try to counter balance US influence throughout the region,” says John Nagl, a counterinsurgency expert and president of the Center for a New American Security, a Washington-based think tank.

Weapons stockpiling

Buchanan says weapons used in attacks include Iranian-made rockets manufactured as recently as 2010 representing an increase in munitions coming across the Iranian border over the past eight months.

At a recent display for a small group of journalists last week at one of the military’s most secretive installations, military explosives experts displayed rockets, powerful roadside bombs, and timing devices recovered in attacks on US forces. The arms included an improvised rocket-assisted munition – known as an IRAM – used extremely effectively in attacks against American forces in the south. Only Iranian-linked groups use the mortars, made more lethal by attaching warheads from rockets, say officials.

[Edtior's Note: An earlier version of this story incorrectly described an IRAM, an improvised rocket-assisted munition].

US explosive experts linked a fatal attack on US soldiers in June to the Iranian-backed group Kitaib Hizbollah. An explosives expert says fingerprints on the truck used in the rocket attack led them to a suspected militant who had been in US custody previously.

The explosives experts, who insisted on not being identified by name, would talk in only the most general terms on what features identify the rockets and bombs as having been made in Iran. Iranian officials deny arming or training militants in Iraq.

Buchanan said the Iraqi Army has found significant stockpiles of rockets and improvised explosive devices, including a cache of 49 fully ready EFPs – the most lethal roadside bombs – discovered in a Baghdad neighborhood two months ago but has proved less capable of stopping the supply lines.

“What we have not seen a lot of is intercept of any of these munitions while they’re in transit,” Buchanan says. He says many of the shipments seem to be coming across the legal border crossings, including in buses of Iranian religious pilgrims that are not checked by border authorities, he says.

Sadr's powerful position

Despite media reports of a major Iraqi government crackdown on southern Iraq's Shiite militias last month said to involve 2,000 Iraqi soldiers, they have not made significant inroads, according to US officials.

Analysts say that while Maliki significantly boosted his popularity in 2008 by sending in the Iraqi Army into Basra to retake the city from Shiite cleric Moqtada al-Sadr’s Mahdi Army militia, he is too reliant now on Mr. Sadr’s political support to do the same thing.

Sadr is in Iran where he is pursuing his religious studies but his political wing forms an essential part of Maliki’s coalition government while another of his militias, the Promised Day Brigade, is one of the groups blamed for attacks on US forces. Maliki emerged from elections 1-1/2 years ago with fewer seats than his nearest rival and managed to take power only by cobbling together a broad coalition.

“As long as there is this big unwieldy government, it is very unlikely that Maliki is going to turn on the militias the way the United States wants him to do,” says Reidar Visser, an Iraq expert at the Norwegian Institute of International Affairs who also maintains Historiae.org.

The US has given assurances to Iraq it will not attack Iran from Iraqi territory. Despite the covert attacks, Iran is also believed to have assured Iraq it will not openly attack US forces.

“America accuses Iran and Iran accuses America. At the end, this is part of two decades of struggle between America and Iran,” says Abbas al-Bayati, a member of parliament from Maliki’s State of Law coalition. “But there are also indirect side agreements. If we didn’t have such agreements, Iraq now would be hell.”