Iran said on Sunday it is fully prepared to enrich uranium at any level and with any amount, in further defiance of US efforts to squeeze the country with sanctions and force it to renegotiate a 2015 nuclear deal with world powers.

In a news conference broadcast live, senior Iranian officials said Tehran would keep reducing its commitments every 60 days unless signatories of the pact moved to protect it from US sanctions.

"In a few hours the technical process will come to an end and the enrichment beyond 3.67 percent will begin," said Behrouz Kamalvandi, the spokesman for Iran's Atomic Energy Organisation.

"And tomorrow early in the morning, when the IAEA (UN nuclear watchdog) takes the sample we would have gone beyond 3.67 percent."

Under the pact, Iran can enrich uranium to 3.67 percent fissile material, well below the 20 percent it was reaching before the deal and the roughly 90 percent suitable for a nuclear weapon.

Kamalvandi said Iran would enrich uranium for use in fuelling its Bushehr power plant, to the level of 5 percent, confirming what Reuters reported on Saturday.

"We are fully prepared to enrich uranium at any level and with any amount," he said.

Iran shows no sign of caving in to pressure from US President Donald Trump in a confrontation that has taken on a military dimension, with Washington blaming Tehran for attacks on oil tankers, and Iran shooting down a US drone, prompting aborted US air strikes.

Long-tense relations between Tehran and Washington took a turn for the worse in May 2018 when Trump pulled out of the 2015 nuclear deal reached before he took office, and reimposed sanctions.

Trump argues that the deal is too weak because some of its terms are not permanent, and because it does not cover non-nuclear issues such as Iran's ballistic missile programme and regional aspirations.

Iran's announcement challenging Washington is a test of European diplomacy. The Europeans, who opposed last year's decision by Trump to abandon the agreement, had pleaded withIran to keep within its parameters.

Tehran has expressed frustration over what it says is the failure of European parties to the agreement to salvage the pact by protecting Iran's economic interests from US sanctions.