“We are ready to help Syria on all grounds to confront threats,” Mohammad Reza Aref, the Iranian VicePresident, said after meeting Naji al-Otari, the Syrian Prime Minister, in Tehran.

“This meeting, which takes place at this sensitive time, is important, especially because Syria and Iran face several challenges and it is necessary to build a common front,” Mr al-Otari said.

Neither country elaborated on what the common front would entail, though Iranian state television said that Tehran would share with Syria its experience of dealing with sanctions. But the two countries, positioned on either side of Iraq, have enormous capacity to deepen the chaos in that country, cause further trouble in Lebanon and sponsor terrorist attacks abroad.

The White House responded by sharply reminding both states that they had “international obligations and needed to abide by the commitments they have made to the international community”.

The rising tensions in the region were amply demonstrated when Iranian television reported a powerful explosion near the Iranian port of Deylam, in the Gulf. Witnesses claimed to have seen an unidentified aircraft firing a missile.

Financial markets plunged and oil prices rose amid fears that Israel had launched a pre-emptive strike on the Bushehr nuclear facility 100 miles away, as it did against the Iraqi Osirak nuclear reactor in 1981.

By nightfall the Iranian military was saying that the explosion was connected to the construction of a dam, although that was merely the latest of several explanations.

Early in the day, before the explosion, Tehran had accused the US of using drones and other aircraft to spy on Iran’s nuclear and military installations and promised to shoot them down. “We have the means to hit them and if they get near, our anti-aircraft defence systems will attend to it,” Ali Younessi, the Iranian Intelligence Minister, said.

Washington has long accused Syria of harbouring terrorists and assisting the insurgency in Iraq, but it has sharply increased its verbal attacks on Syria since the assassination on Monday of Rafik Hariri, the former Lebanese Prime Minister, in Beirut.

Hundreds of thousands of Lebanese mourners crowded into Beirut’s central square yesterday, turning Mr Hariri’s funeral into a mass demonstration against Syrian domination of its small neighbour.

The US used the event to demand the immediate withdrawal of Syria’s 15,000 troops from Lebanon. The death of Mr Hariri “must give renewed impetus to achieving a free, independent and sovereign Lebanon. What that means is the complete and immediate withdrawal by Syria of all of its forces in Lebanon,” Nicholas Burns, the Assistant Secretary of State, who represented President Bush at the funeral, said.

Mr Burns also said that Washington would be watching Lebanon’s elections this spring for signs of Syrian meddling: “The Lebanese people must be allowed to make their own political choices and to conduct those elections on their own, free of foreign interference and intimidation.”

The issue that is driving the US and Iran closer to confrontation is Tehran’s alleged pursuit of nuclear weapons.

In Washington yesterday, Porter Goss, the Director of the CIA, told Congress that Iran was stepping up efforts to build long-range missiles.

In London, Silvan Shalom, the Israeli Foreign Minister, said that Iran would know within six months how to build a nuclear bomb. “They are trying very hard to develop the nuclear bomb,” he said. “This kind of extreme regime with a nuclear bomb is a nightmare, not only for us. The question is not if the Iranians will have a nuclear bomb in 2009, 2010 or 2011. The main question is when are they going to have the knowledge to do it. We believe in six months they will end all the tests and experiments they are doing to have that knowledge.”

Iran insists that its pursuit of nuclear know-how is confined to energy production, but the US believes that the projects are cover for its ambitions for atomic weaponry.

Syria’s decision to stand so publicly shoulder-to-shoulder with Iran surprised no one in the Bush Administration. For years, US officials have accused Syria of complicity in Iran’s funding of, and support for, Hezbollah terrorists who target Israel.

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