US President Donald Trump said he was prepared to take military action to stop Tehran having a nuclear bomb but left open whether he would sanction the use of force to protect Gulf oil supplies.

Fears of a confrontation between Iran and the US have mounted since Washington blamed long-time foe Iran for Thursday’s attacks on two oil tankers near the strategic Strait of Hormuz shipping lane.

Tehran denies responsibility but the incidents, and similar attacks in May, have further soured relations that have plummeted since Trump pulled the US out of a landmark international nuclear deal with Iran in May of last year.

Since exiting the accord, which gave Iran sanctions relief in return for curbs on its nuclear programme, Trump has restored and extended US sanctions. That has forced countries around the world to boycott Iranian oil or face sanctions of their own.

But in an interview with Time magazine, Trump, striking a different tone from some Republican lawmakers who have urged a military approach to Iran, said last week’s tanker attacks in the Gulf of Oman had had only a “very minor” impact so far.

Asked if he would consider military action to prevent Iran from obtaining nuclear weapons or to ensure the free flow of oil through the Gulf, Trump said: “I would certainly go over nuclear weapons, and I would keep the other a question mark.”

The 2015 nuclear deal with Iran aimed to head off any pathway to an Iranian nuclear bomb but Trump says the deal failed to address Iran’s missile programme or punish it for waging proxy wars in West Asian countries.

Tehran has decried the toughening of US sanctions and urged other signatories to take action to save the nuclear pact or see Iran turn its back on the deal.

Iran said on Monday it would soon breach limits on how much enriched uranium it can stockpile under a 2015 nuclear deal, which had sought to limit its nuclear capabilities. Exceeding the uranium cap at the heart of the accord could prompt a diplomatic crisis, forcing the other signatories, which include China, Russia and European powers, to confront Iran.

The White House National Security Council condemned the statement as “nuclear blackmail”.

Russia told the US it should drop what it called provocative plans to deploy more troops to West Asia and cease actions that looked like a conscious attempt to provoke war with Iran, and urged restraint on all sides.

“What we see are unending and sustained US attempts to crank up political, psychological, economic and yes military pressure on Iran in quite a provocative way,” deputy foreign minister Sergei Ryabkov was cited as saying by Russian media.

“They (these actions) cannot be assessed as anything but a conscious course to provoke war,” he said.

Acting US defence secretary Patrick Shanahan on Monday announced the deployment of about 1,000 more troops to West Asia for what he said were defensive purposes, citing concerns about a threat from Iran. The deployment is in addition to a 1,500-troop increase announced last month.

Iranian President Hassan Rouhani said in a speech that Iran did not seek war and dismissed US efforts to isolate Iran. “Iran will not wage war against any nation,” he said. “Despite all of the Americans' efforts in the region and their desire to cut off our ties with all of the world and their desire to keep Iran secluded, they have been unsuccessful.”