WASHINGTON — US President Barack Obama Thursday demanded answers from the pinnacle of Iran's government over an alleged plot to kill the Saudi envoy to Washington and said the facts of the plan were not in dispute.

But he also declined to say whether US officials believed that the alleged scheme was endorsed at the very highest levels of the Iranian regime, though ascribed it to a pattern of "dangerous and reckless" behavior by Tehran.

"We believe 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," Obama said.

"The important thing is for Iran to answer the international community, why anybody in their government is engaging in these kinds of activities?"

State Department spokesman Victoria Nuland, meanwhile, said that Washington had been in "direct contact" with Tehran over the plot, adding the interaction took place on Wednesday and outside Iran.

A senior State Department official told reporters later on the condition of anonymity that the contact took place at the US initiative.

Iran has strongly denied any involvement in what the US says was a plot by the Iranian Revolutionary Guards' elite Quds force to kill the ambassador by hiring assassins from a Mexican drug cartel for $1.5 million.

Questions are also mounting about why Tehran would countenance such an explosive act and over the details of the scheme, and whether it was perhaps a renegade operation by Quds force officers.

But Obama, answering questions about the alleged plot for the first time, in a White House news conference, insisted the "facts are there for all to see."

"We would not be bringing forward a case unless we knew exactly how to support all the allegations that are contained in the indictment," Obama said.

"We believe that after people have analyzed (the details) there will not be a dispute that this is in fact what happened."

Obama said his government had offered evidence of the alleged operation to its allies, to establish the genuine nature of the plot and to empower an effort to further punish and isolate Iran.

"What we know is that an individual of Iranian-American descent was involved in a plot to assassinate the ambassador to the United States from Saudi Arabia," he said.

"We also know that he had direct links, was paid by and directed by individuals in the Iranian government."

Washington has named two principal suspects in the case: Manssor Arbabsiar, a used-car salesman who is a naturalized US citizen, and Gholam Shakuri, said to be an Iran-based Quds force operative.

Shakuri is believed to be in Iran, while Arbabsiar, 56, was arrested on September 29 at New York's John F. Kennedy airport.

Obama said that the alleged plot was part of a pattern of "dangerous and reckless behavior by the Iranian government," and promised effective US measures in response, including prosecutions and actions targeting Tehran.

He said Washington would apply "the toughest sanctions" and "continue to mobilize the international community to make sure that Iran is further and further isolated and that it pays a price for this kind of behavior."

Senior Treasury Department official David Cohen said the United States was considering further sanctions against Iran's central bank in response to the alleged plot.

The Treasury has already slapped sanctions on five people linked to one branch of Iran's military and against the country's second-largest air carrier over the plot.

Earlier, Iran urged Saudi Arabia "not to fall into the trap" of believing US claims it was behind a scheme which it said served only the United States and Israel.

"I am asking Saudi Arabia not to fall into the trap, because any disturbance in relations between countries in the (Middle East) region will only benefit the United States and the Zionist regime," said Ali Ahani, Iran's deputy foreign minister in charge of Europe and America affairs, according to the IRNA news agency.

Obama on Wednesday called Saudi King Abdullah to talk about the alleged plot, which has further hiked tensions in the Gulf, and to vow a united response.