LONDON (Reuters) - Iran urged the United States on Monday to address the Islamic Republic with respect, not threats of war, a day after U.S. President Donald Trump stoked concerns about a potential U.S.-Iran conflict.

FILE PHOTO: Iranian Foreign Minister Mohammad Javad Zarif in Tokyo, Japan, May 16, 2019. REUTERS/Kim Kyung-Hoon/File Photo

But in a sign of brewing confrontation a year after Washington quit world powers’ 2015 nuclear deal with Iran and reimposed sanctions on it, Tehran announced a fourfold increase in its rate of production of low-grade uranium enrichment.

Tensions between Washington and its Sunni Muslim Gulf Arab allies on one side and Tehran and its Shi’ite Muslim proxies in the region on the other have been flaring for weeks.

On Sunday, Trump tweeted: “If Iran wants to fight, that will be the official end of Iran. Never threaten the United States again!”

Iranian Foreign Minister Mohammad Javad Zarif replied on his twitter account “NeverThreatenAnIranian. Try respect—it works!”

Zarif, who was educated in the United States, actually praised Trump for earlier remarks seen as cautioning hawks in his administration who were encouraging conflict.

The president “rightly deplores ‘military-industrial complex’ pushing U.S. #ForeverWars,” Zarif wrote on Twitter.

But he said Trump had allowed a “B-team” of aides led by National Security Advisor John Bolton to “trash diplomacy”. He accused them of “milking despotic butchers via massive arms sales”, an apparent reference to Iran’s main regional foe, Saudi Arabia, Washington’s biggest arms buyer.

Trump has tightened economic sanctions against Iran, and his administration says it has built up the U.S. military presence in the region. It accuses Iran of posing threats to U.S. troops and interests. Tehran has denied this, describing U.S. moves as “psychological warfare” and a “political game”.

Britain told Iran on Monday not to underestimate the resolve of the United States, warning that if American interests were attacked then the Trump administration would retaliate.

The foreign minister of Oman, a Gulf Arab state that in the past helped pave the way for negotiations between Iran and the United States, visited Tehran on Monday. Yousuf bin Alawi bin Abdullah addressed regional and international issues with Zarif, Iranian state news agency IRNA said, without elaborating.

NUCLEAR DEAL

Quoting an official at the Natanz enrichment plant, the semi-official Tasnim said Iran was accelerating the rate of production at which it refines uranium to 3.67% fissile purity, suitable for civilian nuclear power generation.

The move came two weeks after Iran, acting after Trump reimposed sanctions aimed at blocking all Iranian oil exports to cripple its economy, declared it would scale back some commitments under the accord it signed with six world powers.

Under the 2015 deal, the Islamic Republic was allowed to stockpile a maximum of 300 kg of low-enriched uranium, and ship any excess out of the country for storage or sale.

Iran said this month that cap no longer applied in response to the unilateral U.S. withdrawal from the deal.

Trump has condemned the accord, signed by his predecessor Barack Obama, as flawed for not being permanent and for not covering Iran’s ballistic missile programme and role in conflicts around the Middle East.

Behrouz Kamalvandi, spokesman for the Atomic Energy Organization of Iran, told Tasnim that the U.N. atomic watchdog, the International Atomic Energy Agency (IAEA), had been notified about the move to step up the production rate of low-enriched uranium fourfold.

“It won’t be long until we pass the 300-kg limit of low enriched uranium. So it’s better for the other side to do what it’s necessary to be done,” Kamalvandi said, alluding to steps by other powers to shield Iran’s economy from U.S. sanctions.

He said the acceleration of Iran’s centrifuge enrichment machines remained within the bounds of the nuclear deal, and Tehran had no intention to exit the accord.

It was not clear how far Iran’s current stock of low-enriched uranium was from the 300-kg limit.

Under the deal Iran may enrich uranium to 3.67% purity - far below the 90% of weapons grade, and also below the 20% level to which Iran refined uranium before the deal.

Iran has threatened that in 60 days it would resume enrichment beyond the 3.67% level unless remaining signatories of the deal - Russia, China, Britain, France and Germany - find a way to safeguard its vital oil and banking sectors.