President Donald Trump has dramatically increased pressure on Tehran, withdrawing from an international agreement aimed at ending its nuclear programme and introducing several rounds of unilateral US sanctions.

The latest tranche of measures have been touted as the toughest yet, and aim to significantly reduce Iran's vital oil exports and cut off its banks from international finance.

Speaking in Singapore ahead of a summit, Bolton said: "We think the government is under real pressure and it's our intention to squeeze them very hard.

"As the British say, squeeze them until the pips squeak.

"We are also going to significantly increase the enforcement of sanctions."

The US measures have been opposed by other parties to the deal aimed at ending Iran's nuclear drive -- Britain, France, Germany, China and Russia -- who have vowed to keep the accord alive.

"There's no doubt Iran has already started to try and find ways to evade the sanctions both on oil in particular and financial markets," Bolton added.

UN inspectors say Iran is abiding by the agreement.

Bolton, however, said that "most of the countries of Europe have passed through denial and anger and many of them are already at acceptance that we're out of the deal.

"Others are getting to that point."

In the face of widespread opposition to the sanctions, Washington issued exemptions to eight countries to its ban on buying oil from major producer Iran.

Washington is demanding that Tehran end policies rooted in the 1979 Islamist revolution, including its support for regional proxies such as the Lebanese militia Hezbollah and its development of missiles.

The only support for the US position has come from Iran's regional rivals, notably Saudi Arabia and Israel.

The International Monetary Fund has forecast that the sanctions will cause Iran's economy to contract 1.5 percent this year and 3.6 percent next year.