President Barack Obama on Thursday vowed to seek the “toughest sanctions” against Iran after the U.S. discovered a plot to assassinate the Saudi ambassador in Washington.

The alleged plot is “not just a dangerous excalation, this is part of a pattern of dangerous and reckless behavior by the Iranian government,” he said.

Iran has denied any part in the plot.

Obama was speaking at a joint press conference with South Korean President Lee Myung-bak.

He said the U.S. would work with international partners so make sure the country “pays a price.”

U.S. Attorney General Eric Holder on Tuesday said two individuals had been charged in the matter.

A complaint filed in federal court charges manssor Arbabisiar, a 56-year old naturalized U.S. citizen who both Iranian and U.S. passports, and Gholam Shakuri, an Iran-based member of Iran’s Qods Force, a special operation’s unit within the Iranian Islamic Revolutionary Guard Corps.

Both defendants’ charges are related to a murder-for-hire plot involving explosives.

Shakuri remains at large, the Justice Department said Tuesday.

Arabisar faces life in prison.

Holder said the plot – to bomb a restaurant the Ambassador Adel A. Al-Jubeir frequented - was disrupted before anyone was harmed.

“As alleged, these defendants were part of a well-funded and pernicious plot that had, as its first priority, the assassination of the Saudi Ambassador to the United States, without care or concern for the mass casualties that would result from their planned attack,” said U.S. Attorney Preet Bharara.

The pair had been plotting since the spring of this year, officials said.

Arbabisiar allegedly met with a condfidential Drug Enforcement Agency source in Mexico posing as an associated of a drug cartel and had planned to hire the source for $1.5 million to murder the Ambassador.

U.S. authorities allege Arabisar had no concern for killing innocents.

“They want that guy [the Ambassador] done [killed], if the hundred go with him f**k ‘em,” Arabisar allegedly said, according to the Justice Department Complaint.

The complaint alleges Arbabsiar, with Shakuri’s approval caused two overseas wire transfers totaling approximately $100,000 to be sent to an FBI undercover account as a down payment for CS-1 to carry out the assassination. The remaining funds would allegedly be paid after the attack.

Arbabsiar was arrested on September 29, 2011 during a layover flight at JFK international Airport in New York.

Arbabsiar allegedly confessed to authorities, waving his right to remain silent, the complaint states. Arbabsiar said he was recruited by men he understood to be senior officials in Iran’s Qods Force.