WASHINGTON - US President Barack Obama on Thursday vowed to apply the "toughest sanctions" on Iran and to garner international support to further isolate the Islamist state in the wake of the revelation of an alleged plot to assassinate Saudi ambassador to Washington.

What "we're going to continue to do is to apply the toughest sanctions and continue to mobilize the international community to make sure that Iran is further and further isolated and pays a price for this kind of behavior," Obama said at the joint press conference with visiting South Korean President Lee Myung-bak.

"This is not just a dangerous escalation," said Obama. "This is part of a pattern of dangerous and reckless behavior by the Iranian government."

He said Iran's involvement in the plot indicates "the degree to which it has been outside of accepted norms of international behavior for far too long."

"This is just one example of a series of steps that they've taken to create violence and to behave in a way that you don't see other countries doing," Obama added.

Two men were charged by the US Justice Department on Tuesday for their alleged participation in a plot to assassinate the Saudi ambassador to the United States. The two suspects are Manssor Arbabsiar, a 56-year-old naturalized US citizen holding both Iranian and US passports, and Iranian Gholam Shakuri.

US alleged that the plot was directed by senior members of the Quds Force, an elite division of Iran's Revolutionary Guard Corps responsible for foreign operations. US Secretary of State Hillary Clinton on Wednesday vowed to hold Iran accountable for its alleged involvement in the plot.

Obama warned that Iran must stop its dangerous behavior and there are going to be consequences to its actions, adding that all options are on the table with regard to how the US deals with Iran.

"We will continue to apply the sorts of pressure that will have a direct impact on the Iranian government until it makes a better choice in terms of how it is going to interact with the rest of the international community," he said.

After the plot was made public, the US Treasury Department has slapped sanctions on individuals, including four senior officers from the Quds Force, and Mahan Air, a Tehran-based commercial airline. The US is also urging other countries to follow suit in imposing new sanctions on Iran.