Britain has pledged backing for US reprisals against Iran following an apparent assassination plot in Washington.
Downing Street said Tehran should be "held accountable" amid allegations that government factions were involved in a plan to kill the Saudi ambassador.
The American authorities announced that two Iranian nationals had been charged with offences including conspiracy to murder a foreign official.
Manssor Arbabsiar, who is in custody in New York, and his alleged co-conspirator Gholam Shakuri, who is said to be in Iran, are believed to have been targeting a restaurant frequented by ambassador Adel Al-Jubeir.
The plot was apparently uncovered by a US agent posing as part of a Mexican drugs cartel, who was enrolled by the suspects to help.
US attorney general Eric Holder told a press conference: "The criminal complaint unsealed today exposes a deadly plot directed by factions of the Iranian government to assassinate a foreign ambassador on US soil with explosives. Through the diligent and co-ordinated efforts of our law enforcement and intelligence agencies, we were able to disrupt this plot before anyone was harmed."
Secretary of state Hillary Clinton said the US was preparing new penalties against Iran, including financial sanctions against individuals.
A Downing Street spokesman said: "We congratulate the US authorities on the successful operation to disrupt a conspiracy to attack diplomats in the United States.
"Indications that this plot was directed by elements of the Iranian regime are shocking. Directing an attack against another country is a flagrant violation of international law which calls for the strongest condemnation. The United Kingdom is in close touch with the US authorities on this case. We will support measures to hold Iran accountable for its actions."
Tehran has reportedly dismissed the allegations, with a spokesman for president Mahmoud Ahmadinejad branding them a "fabrication".