US-Iranian tensions have mounted significantly in the past few days, with heightened rhetoric on both sides and the US decision to establish a military base in Iraq less than five miles from the Iranian border to block the smuggling of Iranian arms to Shia militias.

The involvement of a few hundred British troops in the anti-smuggling operation also raises the risk of their involvement in a cross-border clash.

US officers have alleged that an advanced Iranian-made missile had been fired at an American base from a Shia area, which if confirmed would be a significant escalation in the "proxy war" referred to this week by General David Petraeus, the US commander in Iraq.

"The proxy war that has been going on in Iraq may now cross the border. This is a very dangerous period," Patrick Cronin, the director of studies at the International Institute for Strategic Studies, said.

Iran's leaders have so far shown every sign of relishing the confrontation. The supreme leader, Ayatollah Ali Khamenei, declared yesterday that American policies had failed in the Middle East and warned: "I am certain that one day Bush and senior American officials will be tried in an international court for the tragedies they have created in Iraq."

In such circumstances, last week's Israeli air strike against a mystery site in northern Syria has triggered speculation over its motives. Israel has been silent about the attack. Syria complained to the UN security council but gave few details. Some say the target was Iranian weapons on their way to Hizbullah in Lebanon, or that the sortie was a dry run for a US-Israeli attack on Syria and Iran. There is even speculation that the Israelis took out a nuclear facility funded by Iran and supplied by North Korea

The situation is particularly volatile because the struggle for influence threatens to exacerbate a confrontation over Tehran's nuclear ambitions.

The US has called a meeting of major powers in Washington next Friday to discuss Iran's defiance of UN resolutions calling for its suspension of uranium enrichment. It comes amid signs that the Bush administration is running out of patience with diplomatic efforts to curb the nuclear programme. Hawks led by the vice-president, Dick Cheney, are intensifying their push for military action, with support from Israel and privately from some Sunni Gulf states.

"Washington is seriously reviewing plans to bomb not just nuclear sites, but oil sites, military sites and even leadership targets. The talk is of multiple targets," said Mr Cronin. "In Washington there is very serious discussion that this is a window that has to be looked at seriously because there is only six months to 'do something about Iran' before it will be looked at as a purely political issue."

US presidential elections are due in November 2008, and military action at the height of the campaign is usually seen by voters as politically motivated.

Vincent Cannistraro, a former CIA counter-terrorism chief who is now a security analyst, said: "The decision to attack was made some time ago. It will be in two stages. If a smoking gun is found in terms of Iranian interference in Iraq, the US will retaliate on a tactical level, and they will strike against military targets. The second part of this is: Bush has made the decision to launch a strategic attack against Iranian nuclear facilities, although not before next year. He has been lining up some Sunni countries for tacit support for his actions."

US and British officials have complained to Iran about the use by Shia militias in Iraq of what they say are Iranian-made weapons. The main concern is the proliferation of roadside bombs that fire a bolt of molten metal through any thickness of armour, which the officials say must have been made in Iran.

A US military spokesman in Baghdad, Major General Kevin Bergner, raised the stakes when he said the 240mm rocket that hit the US military headquarters outside Baghdad this week, killing an American soldier and wounding 11, had been supplied to Shia militants by Iran.

Gen Bergner used to work in the White House, where he was aligned with administration hawks, and his dispatch to Baghdad was seen by some as a move to increase pressure on Iran.

"There are an awful lot of lower level officers who are very angry about the deaths from explosively formed projectiles said to come from Iran. There is a certain amount of military pressure to do something about this," said Patrick Clawson, the deputy director for research at the Washington Institute for Near East Policy. "That said, it is very difficult for us to do anything without much better evidence. In that respect, border control is a sensible solution."

Any US decision to attack Iran would force Gordon Brown to choose between creating a serious rift in the transatlantic alliance and participating in or endorsing American actions. British officials insist that Washington has given no sign it is ready to abandon diplomacy and argue that UN sanctions are showing signs of working. They point to the resurgence in Iran of Hashemi Rafsanjani, seen as a pragmatic counterweight to President Mahmoud Ahmadinejad.

Hopes that a new war could still be avoided have also been boosted by Gen Petraeus's claim that Iran's covert Quds force alleged to be supporting Shia attacks on coalition forces had been pulled out of Iraq. If true, it could be that in the stand-off between the US and Iran, Iran has blinked first.