MATT SPETALNICK

Iran is a destabilising force in Iraq, George W Bush insisted yesterday, despite Tehran's assertion to Iraqi Prime Minister Nuri al Maliki that it was helping secure his country.

Calling Iran a "very troubling nation" that should be isolated, Bush warned at a White House news conference: "When we catch you playing a non-constructive role in Iraq, there will be a price to pay."

Bush spoke as al Maliki, facing deepening political woes at home and US criticism for lack of progress in bridging sectarian divisions, won pledges of support from Shi'ite Iran during his visit there.

Playing down signs of warming ties between Baghdad and Tehran, Bush - who is struggling to rally US support for the unpopular Iraq war - voiced confidence that he and Maliki saw eye-to-eye on Iran as a threat to Iraqi security.

"If the signal from al Maliki is that Iran is constructive, I will have to have a heart-to-heart with my friend, the prime minister. Because I don't believe they are constructive," Bush said. "I don't think he, in his heart of heart, think they're constructive either."

It was the second time this week that Bush has had to defend his tough stance on Iran in the face of possible differences with a key ally.

He warned Afghan President Hamid Karzai on Monday to be more suspicious of Iran after the Afghan leader had brushed aside US accusations that Tehran was arming the Taliban.

ITehran denies Washington's accusations that it is supplying weapons to militants in Iran to stoke violence, and instead blames the US military presence. Iranian leaders vowed their support for Iraq's security, but in talks with al Maliki stuck by their stance that real peace would only come when US forces pull out.

"Establishment of peace and tranquillity in Iraq depends on withdrawal of occupiers and their avoidance of interfering in Iraq," said Iranian Vice President Parviz Davoodi.

In Iraq itself, hundreds of thousands of Shi'ite pilgrims made their way on foot to a shrine in the north of Baghdad yesterday, praying for safety at an annual rite whose past two years have been marred by violence.

Pilgrims waved flags, chanted and beat their chests in a traditional gesture of ritual mourning. Others carried the symbolic green coffin of Imam Musa Kadhim, a Shi'ite martyr imprisoned and poisoned in Baghdad 1200 years ago.

Many had walked for days from distant towns in intense heat to reach the shrine where Kadhim is buried.

"Of course, I am afraid. But God willing I will come home safe," said Um Khaled, a woman in black robes. "I have made this pilgrimage every year, it will not stop me," she said of the threat of violence.

There were far fewer reports of violence than on a typical Baghdad day. A roadside bomb in southern Baghdad killed three people. Three soldiers were wounded by gunmen in an attack that police earlier had said hit pilgrims.

Two years ago, nearly 1000 pilgrims were killed in a stampede on a bridge near the shrine sparked by rumours of a suicide bomber, the single most deadly incident since the US-led invasion to topple Saddam Hussein in 2003.

Last year, gunmen, some on rooftops, ambushed pilgrims on the way to the shrine, killing 20 and wounding 300.

In the US, all charges were dropped yesterday against a US Marine accused of killing three Iraqis in Haditha - a massacre that resulted in the biggest US criminal case involving civilian deaths to come out of the Iraq war.

Marine Lance Corporal Justin Sharrat was accused of killing three Iraqi brothers in response to a 2005 roadside bombing in the Iraqi city.

Military authorities have said that after the bombing, a group of marines went on a rampage, killing 24 Iraqi men, women and children.