Tehran: Iran on Saturday hardened its denial of involvement in a thwarted assassination plot in Washington, with its supreme leader slamming the US accusations as "absurd" and without effect.

The remarks - the first direct response by Ayatollah Ali Khamenei -- came as Iran's government again urged Saudi Arabia to be wary over the plot claims that Iranian officials tried to contract a Mexican drug cartel to kill the Saudi ambassador to Washington.

The US government is rallying other nations to its bid to further isolate Iran over the plot and has provided allies with details it says prove the outlandish assassination bid was genuine.

US President Barack Obama on Thursday warned Iran would be held to account over the alleged plot, which he described as part of a pattern of "dangerous and reckless" behaviour by Tehran.

Iran's strenuous denials of involvement, made ever since the alleged plot was made public on Tuesday, were capped on Saturday with Khamenei's speech.

"It's a meaningless and absurd accusation regarding a number of Iranians," he said in a speech carried by state television. "But it has not stuck and it will not stick," he said.

Khamenei added defiantly, "They say that they want to isolate Iran. They are the ones who are isolated."

US officials admit they do not know which of Iran's leaders, if any, are implicated.

But Obama stressed that "even if at the highest levels there was not detailed operational knowledge, there has to be accountability with respect to anybody in the Iranian government engaging in this kind of activity."

The US Justice Department and FBI say the trail leads to officials inside the Quds Force, a special operations outfit within Iran's elite Revolutionary Guards.

Quds Force personnel are said to have transferred nearly USD 100,000 to the bank account used by a member of a Mexican drug cartel who was really a paid US informant who tipped off the FBI.

The money was allegedly a down payment for a USD 1.5-million hit on the Saudi ambassador, Adel al-Jubeir, possibly through the bombing of a Washington restaurant.

An Iranian used-car salesman who is a naturalised US citizen living in Texas, Manssor Arbabsiar, is said to have confessed to acting as the go-between for his cousin, whom he
described as a high-ranking official in the Quds Force, and the Mexican cartel.

Arbabsiar is in US custody, charged with the plot.