The US embassy in London and lord mayor Boris Johnson were among a high-profile list of potential targets of a suspected terror cell in Britain.
The stock exchange, Big Ben and the Palace of Westminster were also on the hit list, along with named religious leaders.
The plot details were made public after nine men accused of planning a series of Christmas bomb attacks in the British capital were remanded in custody late on Monday to appear at the Old Bailey on January 14.
The men, aged between 19 and 28 and with roots in Britain’s Bangladeshi community, were named as Mohammed Chowdhury, 20, and Shah Rahman, 28, from East London; Gurukanth Desai, 28, Abdul Miah, 24, and Omar Latif, 26, all from Cardiff; and Nazam Hussain, 25, Usman Khan, 19, Mohibur Rahman, 26, and Abul Shahjahan, 26, from Stoke, central England.
They were arrested in raids on December 20 and later charged with conspiracy to cause explosions in Britain and other terrorism offences. Three other men were released without charge.
US State Department spokesman Mark Toner confirmed to reporters that officials at the US embassy in London “are aware of this, are working quite closely with British authorities and appreciate the high level of co-operation we have with them and are obviously taking suitable security precautions”.
The US stepped up security procedures at all its overseas missions after an attempted parcel bombing at the Greek embassy in Rome.
“We have notified all US embassies worldwide to review current mail-screening procedures and to continue vigilance when opening mail,” Mr Toner said.
Officials at the Greek embassy in Italy received a package on Monday similar to ones that exploded last week at the Swiss and Chilean embassies there.