In his first public remarks on the assassination scheme, Mr. Obama sought to counter skepticism about whether Iran’s Islamic government directed an Iranian-American car salesman to engage with a Mexican drug cartel to kill Saudi Arabia’s ambassador to the United States and carry out other attacks. Mr. Obama insisted that American officials “know that he had direct links, was paid by, and directed by individuals in the Iranian government.”

“Now those facts are there for all to see,” Mr. Obama said. “We would not be bringing forward a case unless we knew exactly how to support all the allegations that are contained in the indictment.”

The president did not lay out any specific new sanctions against Iran; his administration is considering a number of measures, but has limited leverage and would have to muster international support to impose anything with real teeth.

While Mr. Obama made his remarks during a news conference in the White House East Room with the South Korean president, Lee Myung-bak, the State Department said that United States officials had been in direct contact with the government of Iran over the accusations.

The State Department spokeswoman, Victoria Nuland, would provide no details. But Thursday night a White House official said the contact had been made by the United States ambassador to the United Nations, Susan E. Rice, who gave a letter to her Iranian counterpart, Mohammad Khazaee.