The US says it will increase international pressure on Iran, which it alleges is behind a plot to kill the Saudi ambassador in Washington.Two Iranians have been charged with conspiracy over what US officials said was a plan conceived and directed by Iran\'s Revolutionary Guard Corps.Iran has dismissed the allegations as false and baseless.Meanwhile the US state department issued a worldwide alert, warning of possible anti-US actions.\"The US government assesses that this Iranian-backed plan to assassinate the Saudi ambassador may indicate a more aggressive focus by the Iranian government on terrorist activity against diplomats from certain countries, to include possible attacks in the United States,\" the alert said.It urged Americans residing and travelling abroad to review the information available when making travel plans.US Attorney General Eric Holder suggested there was direct involvement by Iran in the plot.\"The complaint alleges that this conspiracy was conceived, sponsored and directed from Iran and constitutes a flagrant violation of US and international law,\" he said.\"In addition to holding these individual conspirators accountable for their alleged role in this plot, the United States is committed to holding Iran accountable for its actions.\"In a statement, UK Prime Minister David Cameron\'s office said: \"Indications that this plot was directed by elements of the Iranian regime are shocking... We will support measures to hold Iran accountable for its actions.\"US officials have said military action was not being considered.Secretary of State Hillary Clinton said the US would consult with its international partners to send a \"very strong message\" over the alleged plot.\"It was a terrific achievement by our law enforcement and intelligence communities, and we will be consulting with our friends and partners around the world about how we can send a very strong message that this kind of action, which violates international norms, must be ended,\" she said.She said the US was preparing new penalties against Iran, which is already subject to a variety of international sanctions.The US Treasury Department placed five Iranians, including the two men charged, under sanctions on Tuesday for their alleged involvement in the plot.The two accused were named as Manssor Arbabsiar, a 56-year-old naturalised US citizen with dual Iranian and US passports, and Gholam Shakuri, based in Iran and said to be a member of Iran\'s Quds Force, a unit of the Revolutionary Guard Corps.Mr Arbabsiar, who was arrested at New York\'s John F Kennedy airport on 29 September, has confessed to his involvement in the alleged plot, Mr Holder said.A lawyer for Mr Arbabsiar said he would plead not guilty when he was officially indicted.Mr Shakuri was said to be in Iran.US officials said that on 24 May 2011, Mr Arbabsiar made contact with an informant for the US Drug Enforcement Agency, who was posing as a Mexican drug cartel member.Over a series of meetings, it is said that details emerged of a conspiracy involving members of the Iranian government paying $1.5m (£960,000) for the assassination of Saudi ambassador Adel al-Jubeir on US soil.Justice department officials said the initial envisaged target was the Saudi embassy.But in conversations secretly recorded for the US authorities, Mr Arbabsiar also allegedly considered having the ambassador killed at a purported favourite restaurant, despite the possibility of mass casualties.The plot would have been carried out with explosives, Mr Holder said. But he added that no explosives were ever put in place and the public was not in danger.Mr Holder said Mr Arbabsiar, with approval from Mr Shakuri, wired $100,000 to a US bank account for the informant as a downpayment.Mr Arbabsiar and Mr Shakuri have been charged with conspiracy to murder a foreign official, weapons conspiracy, and conspiracy to commit international terrorism charges.Unnamed US officials also told journalists that the Israeli embassy in Washington was also to have been attacked.Iran\'s official Islamic Republic News Agency called the charges a \"propaganda campaign\" by the US government against Tehran.The allegations were \"a comedy show fabricated by America\", Iranian foreign ministry spokesman Ramin Mehmanparast told the semi-official Iranian news agency, Fars.Mr Arbabsiar appeared briefly at a New York City court on Tuesday. He did not enter a plea and was held without bail.He could face a life prison sentence if convicted on all charges, the Department of Justice said.