Iran is committed to its obligations under an international nuclear deal despite the United States withdrawal from the landmark agreement, its foreign minister said on Thursday, calling the reimposition of the US sanctions “unacceptable”.

Foreign Minister Mohammad Javad Zarif made the comments in a meeting with his Japanese counterpart in Tokyo as tensions rise in the Middle East, fuelling concern that the US and Iran are heading for a conflict.

Iran is exercising “maximum restraint in spite of the fact the US withdrew from [the] JCPOA (Joint Comprehensive Plan of Action) last May”, Zarif said at the beginning of his meeting with Japanese Foreign Minister Taro Kono. He was referring to plan of action signed in 2015 by the US, Iran and other countries, under which Iran curbed its uranium enrichment capacity and won sanctions relief in return.

US President Donald Trump withdrew from the agreement last year and is ratcheting up sanctions on Iran, aiming to strangle its economy by ending its international sales of crude oil.

Japan was a major buyer of Iranian oil for decades before the sanctions. An attack on four oil tankers in the Gulf on Sunday, for which no one has claimed responsibility, and Saudi Arabia's announcement on Tuesday that armed drones hit two of its oil pumping stations have compounded worries about war.

The United States withdrew staff from its embassy in Iraq on Wednesday out of apparent concern about threats from Iran, with US sources saying they believe Iran encouraged the attacks on the oil tankers.

Trump is sending an aircraft carrier group, B-52 bombers and Patriot missiles to the Middle East to counter what the US calls a heightened threat from Iran to US soldiers and interests in the region.

“We believe that escalation by the US is unacceptable and uncalled for,” Zarif told Kono in front of reporters before they met privately.

Nevertheless, Iran has relaxed restrictions on its nuclear programme and threatened action that could breach the nuclear deal, although the initial moves do not appear to violate the agreement.

“I'm concerned that the situation in the Middle East is becoming very tense,” Japanese Prime Minister Shinzo Abe told Zarif in a later meeting.

Kono said it was essential to maintain the nuclear agreement and urged Iran to keep implementing it, echoing other countries.

Asian shippers and refiners have put ships heading to the Middle East on alert and are expecting a possible rise in marine insurance premiums after the attacks on the Saudi oil tankers and pipeline facilities, industry sources said on Tuesday.

Trump's Iran moves trigger warnings

Lawmakers from both parties in Congress demanded more information on the White House's claims of rising threats in the Middle East, warning President Donald Trump off a dangerous escalation with Iran.

The top leaders in Congress the so-called Gang of Eight are to receive a classified briefing from the administration on Thursday. But House Speaker Nancy Pelosi said the White House has resisted a wider presentation for all lawmakers, part of what Democrats say is a pattern of stonewalling.

Pelosi said Trump has “no business” moving toward a Middle East confrontation without approval from Congress. “We have to avoid any war with Iran,” she told fellow Democrats in a meeting, according to a person in the room who spoke on condition of anonymity to discuss comments from the private gathering.

Trump himself denied a report on Tuesday that the administration had reviewed a plan to send 120,000 troops. Still, the actions are exposing skepticism in the US and among foreign allies, a legacy of the 2003 invasion of Iraq that was based on false intelligence.

US officials have not publicly provided any evidence to back up claims of an increased Iranian threat.

“Congress has not authorised war with Iran, and the administration, if it were contemplating military action with Iran, must come to Congress to seek approval,” said Sen Bob Menendez of New Jersey, the top Democrat on the Senate Foreign Relations Committee.

He said he had never seen anything like the “non-answers” coming from the administration.

Republicans and even some Democrats who have been briefed said the threats are legitimate. The chairman of Foreign Relations Committee, Sen Jim Risch of Idaho, said that based on the information he received he supports the administration actions, including the repositioning of the USS Abraham Lincoln aircraft carrier to the Gulf.

“The threat is real,” said Democrat Sen. Mark Warner of Virginia, the ranking member of the Senate Intelligence Committee.

Hardliners target Iran's president as US pressure grows

Growing US pressure on Iran has weakened pragmatic President Hassan Rouhani and made his hardliner rivals more assertive at home and abroad, recent developments show.

When he succeeded firebrand leader Mahmoud Ahmadinejad in 2013, Rouhani was seen as an establishment figure who would do little to end Iran's long standoff with the West. Two years later, his administration signed the nuclear deal with six world powers that spurred hopes for wider political change.

Rouhani's authority is now waning as his brother, a key adviser on the 2015 deal, has been sentenced to jail on unspecified corruption charges, a hardliner rival heads the judiciary and his government is under fire for responding too softly to US President Donald Trump's sanctions squeeze.

The president has urged opposing factions to work together and noted limits on his power in a country where an elected government operates under clerical rule and alongside powerful security forces and an influential judiciary.

“How much authority the government has in the areas that are being questioned must be examined,” the presidency's website quoted Rouhani as saying on Saturday, an apparent attempt to fend off public anger at plummeting living standards.

Ebrahim Raisi, who became head of the judiciary in March and is a contender to succeed Supreme Leader Ayatollah Ali Khamenei, retorted that all branches of government had sufficient authority to carry out their duties.

Local media interpreted the statement as a direct rebuke from Raisi, who ran against Rouhani in the 2017 presidential election.

When Rouhani announced last week that Iran would roll back some of its commitments under the international nuclear deal a year after Trump withdrew, the hardliner daily Kayhan newspaper called the move “late and minimal”.

Restrictions on social media, championed by hardline officials and clerics, are putting further political pressure on Rouhani, who promised in his 2017 and 2013 election campaigns to lift such curbs.

The revolutionary guards have used authorities response to heavy flooding in March to criticise the government and promote their effectiveness.

A video of the head of the Guards ground forces lambasting the government after visiting a flood-stricken area in western Iran in early April was widely circulated on social media. “There are a lot of problems. There is no management. No government official has the courage to go there,” Brigadier General Mohammad Pakpour said in the video.