Britain has frozen the assets of five men, including two key suspects in an alleged Iranian plot to kill the Saudi ambassador in the United States, a government spokesman said on Tuesday.

The five targeted included Manssor Arbabsiar, a naturalised US citizen, who was arrested in September over the plot which Iran has dismissed as a fabricated "comedy show".

Gholam Shakuri, an Iranian who has been named by the US authorities as a co-plotter, was also included. He is a fugitive and believed to be in Iran.

"HM Treasury has designated five individuals under the Terrorist Asset-Freezing Act 2010 in relation to the terrorist activities of Iran and the Quds Force," a government spokesman said.

"We will be consulting with partners, including our EU colleagues, Saudi Arabia, and the US, on how to take forward further action," he added.

US officials have said they believe it likely that the head of Iran's elite Quds force knew of the alleged plot to kill Saudi ambassador Adel al-Jubeir.

Britain's Treasury also blacklisted Qasem Soleimani, who it said was a commander in the Iranian Revolutionary Guard Corps. The other two were named as Iranian national Hamed Abdollahi and Abdul Reza Shahlai, who it said lived in Iran.

US authorities said last week that Arbabsiar had paid a US undercover agent posing as a Mexican drug cartel hitman to assassinate the ambassador.

The affair has escalated tensions between the United States and Iran, relations poisoned by decades of mutual suspicion and more recently Western concerns over Iran's nuclear programme.