Washington, Feb 5 (IANS/RIA Novosti): US Senator John McCain drew a cascade of criticism Monday over an incendiary tweet in which he compared Iranian President Mahmoud Ahmadinejad to a monkey.

"So Ahmadinejad wants to be first Iranian in space - wasn't he just there last week?" McCain, a relentlessly vocal critic of the Iranian leader, wrote on his Twitter feed Monday morning.

In the tweet, McCain provided a link to a Reuters article headlined "Iran Launches Monkey Into Space, Showing Missile Progress".

A deluge of angry and critical responses in the Twittersphere ensued, with users accusing the Republican senator from Arizona of racism and behaviour unbecoming of an elected official.

"Nothing like casual racism to start the week," Twitter user Gavin Matthews wrote.

McCain followed up with another Tweet accusing critics of being overly serious.

"Re: Iran space tweet - lighten up folks, can't everyone take a joke?" he wrote.

McCain has been a fervent supporter of hardline policies aimed at weakening Ahmadinejad's power and targeting Tehran's nuclear ambitions.

During his 2008 presidential campaign, in which he ultimately captured the Republican nomination, McCain was asked at an event when the United States would send an "air-mail message to Tehran".

McCain responded with a joke referencing the Beach Boys hit song "Barbara Ann".

"That old Beach Boys' song, 'Bomb Iran?' Bomb, bomb, bomb," he joked.

Iranian media and officials said last week that it successfully sent a monkey into space and secured the simian's safe return to earth. The short flight was portrayed by Tehran as an important step in its plan to eventually send an Iranian into space.