Tensions between Washington and Tehran deepened yesterday after the American military accused five Iranians detained by its forces in Iraq of helping to supply cash, weapons, and training to Shiite militias on behalf of the Iranian regime.

Iran rejected the claims of collusion with Iraq’s Shiite militants and demanded the release of the officials, who were stationed at a liaison office in Irbil, the capital of Kurdish northern Iraq.

A statement from the American military said: “Preliminary results revealed the five detainees are connected to the Iranian Revolutionary Guard Qods Force, an organization known for providing funds, weapons, IEDs [roadside bombs], technology, and training to extremist groups attempting to destabilize the government of Iraq and attack coalition forces.”

Secretary of State Rice, who is touring the Middle East, said America would no longer tolerate Tehran’s support for armed groups in Iraq.

She said over the weekend that President Bush had issued a standing order to the military to target covert Iranian activities.

She said: “I think there is plenty of evidence that there is Iranian involvement with these networks that are making high-explosive [bombs] and that they are endangering our troops, and that’s going to be dealt with.”

Vice President Cheney also warned Iran last night that it should desist from assisting its Shiite allies in Iraq.

Mr. Cheney said: “I think it’s been pretty well-known that Iran is fishing in troubled waters, if you will, inside Iraq. The threat that Iran represents is growing, it’s multi-dimensional, and it is, in fact, of concern to everybody in the region.”

Iran’s Foreign Ministry claimed that the arrested men were working in a consulate and entitled to diplomatic immunity.

The Foreign Ministry’s spokesman, Mohammad Ali Hosseini, said: “Americans should immediately release the five Iranians and pay compensation for the damages they caused to our office in Irbil.”

Iraq’s foreign minister, Hoshyar Zebari, conceded that the office had not yet been granted consular status, but he appealed for America to recognize the political sensitivities involved.

“We can’t change the geographical reality that Iran is our neighbor,” he said. “This is a delicate balance and we are treading a very thin line.”

Officials in Baghdad predicted that the seized Iranians would be released this week, claiming high ranking officials named in intelligence documents had evaded capture.

Allegations that Iranian Revolutionary Guard officers supplied weapons and know-how to Iraqi militias first surfaced 15 months ago after British troops in Basra were killed by a series of roadside bombs designed to penetrate armor plating.

In recent months such devices have killed American soldiers.

Coalition commanders accuse the paramilitary Iranian Revolutionary Guard of providing bomb-making skills and sniper training to Iraqi militias in bases on Iran’s side of the border.

The Madhi army, the biggest militia in central Iraq, started off as an organized rabble but now boasts hundreds of skilled guerrillas, adept in the techniques of urban warfare.

As Shiite groups have gained in strength, the balance of killing in Iraq has tilted.

Iraq’s most senior Sunni politician, Vice President Tariq al-Hashemi, said over the weekend that the majority of victims of the sectarian conflict are from his community.

The president of Syria, Bashar Al-Assad, welcomed his Iraqi counterpart, President Talabani, to Damascus yesterday in the hope of bolstering diplomatic relations between the neighboring countries.

In what was hailed as a landmark visit that came despite the hostile position Mr. Bush has taken on Syria, Mr. Talabani arrived to a warm reception ahead of five days of negotiations over trade and security issues.