Washington: The United States issued a terror warning to its citizens and international diplomats after claiming to have foiled an Iranian plot to assassinate the Saudi ambassador in Washington.

The State Department said on Wednesday the alleged plot "may indicate a more aggressive focus by the Iranian government on terrorist activity against diplomats from certain countries, to include possible attacks on the United States."

"US citizens residing and traveling abroad should review the Department's Worldwide Caution and other travel information when making decisions concerning their travel plans and activities while abroad," it said.

US officials had earlier said that law enforcement had broken up the "chilling" plot planned high up in the Iranian government, in which elite Quds Force operatives sought to acquire explosives from a Mexican drug gang.

Iran denied the allegations, calling them an "evil plot" in a complaint submitted to the United Nations in which Tehran accused the United States and Israel of engineering the murder of Iranian nuclear scientists.

The exchange of accusations was likely to further deepen the rift between Washington and Tehran, already fiercely divided over Iran's nuclear program and support for anti-Israel militant groups in the Middle East.