Iran's supreme leader has dealt a blow to Washington's hopes of building a broad coalition behind an attack on Afghanistan, declaring that his country would not help the US in any way.
Ayatollah Ali Khameini told a group of war veterans that Washington's support for Israel cast doubt on the sincerity of its campaign against terrorism.
"Iran will provide no help to America and its allies in an attack on suffering, neighbouring, Muslim Afghanistan," he said.
In a meeting with a high-level EU delegation visiting Teheran yesterday, however, Iran's President Ali Khatami, sounded a more conciliatory note.
He said that, if the US provided evidence that groups linked to Osama bin Laden were responsible for the attacks on New York and Washington, a military response would be justified under Article 51 of the United Nations Charter.
The EU delegation, headed by Belgium's foreign minister, Mr Louis Michel, welcomed Iran's expressed commitment to combating international terrorism. But the two sides were unable to conceal differences over how to define terrorism.
In an interview with the German magazine Der Spiegel this week, Iran's foreign minister, Mr Kamal Kharrazi, suggested that movements such as Hizbollah or Hamas should not be classified as terrorists.
"Resistance and extremism are two different things. The just freedom struggle of the Palistinian people has nothing to do with the attack on America," he said.
But the External Affairs Commissioner, Mr Chris Patten, said in Teheran yesterday that all countries must unite to condemn terrorist acts, regardless of their perpetrators.
"It is important that all of us recognise that terrorism is always wrong. Murdering innocent men, women and children is never justified.
"There is no such thing as a distinction between a good and a bad terrorist," he said.
Mr Kharrazi warned yesterday against making a link between the Middle East conflict and the attacks on America.
He added that such a move could create a chasm between the West and the Islamic world.
In his speech yesterday, parts of which were broadcast on Iranian television, Ayatollah Khameini accused the US of having double standards where terrorism is concerned because of Washington's support for Israel.
"They expect the entire world to help them because their interests demand. Do you ever care about others' interests?
"These are the characteristics that make America so hated in the world," he said.
EU officials believe that Ayatollah Khameini's remarks yesterday were designed primarily to reassure the Iranian public that Teheran is not moving too close to its long-time adversary, the US.
But they acknowledge that it is unlikely that Iran will provide any assistance to a military operation against Afghanistan.
They hope, however, that in the second phase of a global campaign against terrorism, Teheran will share information that could lead to the break-up of terrorist groups.
The EU delegation left Teheran for Saudi Arabia yesterday afternoon.
It will travel to Egypt today before moving on to Syria, a state that, like Iran, the US regards as a sponsor of terrorism.
The delegation has stressed throughout its tour of the region that the West has no quarrel with Islam and they welcomed a call yesterday by President Khatami for a fresh dialogue between civilisations.
AP adds: The Iranian Supreme Leader, the Ayatollah Ali Khamenei, yesterday spoke to the families of Iran-Iraq war soldiers during a ceremony held, the 21st anniversary of the war, in Tehran.
In his harshest criticism of the US to date, the ayatollah said Washington was "incompetent" and "disgusting" and it should not expect Iran to agree with its unilateral decisions.