US President Barack Obama (R) speaks to Britain’s Prince Charles as they leave Winfield House, in central London, on May 24, 2011. – AFP Photo

LONDON: The White House Tuesday vowed to further “tighten the screws” on Iran after imposing new nuclear sanctions against seven foreign firms in a bid to choke Tehran's commerce with the outside world.

“There is a message to the government of Iran that there is going to be an increasing cost to its failure to live up to its obligations,” said Ben Rhodes, a deputy US national security advisor.

“We are going to be stepping up the pressure -- it is not going to pause.

We are continuing to look for ways to tighten the screws,” Rhodes said in London, during President Barack Obama's state visit to Britain.

Rhodes said the new sanctions imposed Tuesday were a sign to global firms that “business is better done elsewhere.”