US President George W Bush has warned Iran not to destabilise Afghanistan.
He also said Washington expected Tehran to hand over any members of Osama Bin Laden's al-Qaeda network who might have fled across the border from Afghanistan.
His blunt comments reflect US concerns that Iran is trying to challenge the authority of the interim government in Afghanistan, and may be giving safe haven to al-Qaeda leaders fleeing US and allied military troops there.
Iran denies the allegations.
"Iran must be a contributor in the war against terror," Mr Bush told reporters during an Oval Office briefing.
"Our nation, in our fight against terrorism, will uphold the doctrine of either you're with us or against us."
The Iranian Foreign Ministry was swift to reject Mr Bush's warning as "unfounded".
A spokesman denied Iran was trying to interfere in Afghanistan or harbouring al-Qaeda members.
"Afghan people should decide their own future," Hamid Reza Asefi told Reuters news agency.
He added: "Our borders are tightly closed and the Islamic Republic of Iran in no circumstances would let al-Qaeda members, fighters and supporters of Bin Laden enter the country."
Positive early signals
Early in the US-led war in Afghanistan, which followed the 11 September terror attacks on New York and the Pentagon, Iran offered Washington the use of its territory for search-and-rescue missions involving US forces.
Tehran also took part in the talks in Germany that led to the formation of the interim government in Kabul.
"We had some positive signals early in this war from the Iranians," Mr Bush said.
But a spokesman for the governor of the Afghan city of Kandahar has accused Tehran of supplying arms and cash for local commanders in two nearby provinces.
And according to The New York Times, US special forces around Herat report that Iranian agents have threatened or bribed tribal leaders to undermine US-backed programmes.
President Bush said his administration wanted to work with Iran to ensure the stability of Afghanistan, but warned: "If they in any way, shape, or form try to destabilise the government, the coalition will deal with them... in diplomatic ways, initially."
He added: "We would hope that they would continue to be a positive force in helping us to bring people to justice.
"We would hope, for example, they wouldn't allow al-Qaeda murderers to hide in their country.
"We would hope that, if that be the case, if someone tries to flee into Iran, that they would hand them over to us."
"If they're a part of the coalition, they need to be an active member of the coalition."news.bbc.co.uk/hi/english/world/americas/newsid_1753000/1753521.stmE-mail this article