WASHINGTON (REUTERS, AFP) - US Secretary of State Mike Pompeo said on Monday (April 22) the United States is not extending any waivers exempting importers of Iranian oil from US sanctions and there will be no grace period for those economies to comply.

"We're going to zero. We're going to zero across the board," Mr Pompeo told reporters after the White House announced the end to waivers in order to pressure Iran over its nuclear programme. "There are no (oil) waivers that extend beyond that period, full stop."

US President Donald Trump later said that Saudi Arabia and other OPEC nations could "more than make up" for any drop in Iranian oil supplies to global markets now that his administration has decided to end waivers that allowed some nations to buy Iranian oil without facing US sanctions.

"Saudi Arabia and others in OPEC will more than make up the Oil Flow difference in our now Full Sanctions on Iranian Oil," Mr Trump said on Twitter on Monday.

Mr Pompeo also vowed that the United States will keep fighting "radical Islamic terror" after devastating attacks in Sri Lanka that targeted churches on Easter Sunday and hotels.

"Radical Islamic terror remains a threat. We are continuing to do real work against these evil human beings," Mr Pompeo told reporters.

"This is America's fight, too," added Mr Pompeo, who said he had spoken by telephone with Sri Lanka Prime Minister Ranil Wickremesinghe.

Asked if the threat showed the continued risks from the Islamic State in Iraq and Syria (ISIS) group, Mr Pompeo said the US had succeeded in destroying the movement's self-styled caliphate in Syria but needed to "remain active and vigilant" around the world.

"Sadly, this evil exists in the world," he said.