Iran's Ayatollah Ali Khamenei has the final say in all major policies under Iran's dual system. (File)

Iran will not negotiate with the United States, Supreme Leader Ayatollah Ali Khamenei said on Wednesday, after President Hassan Rouhani signalled talks with Washington might be possible if sanctions were lifted.

Washington withdrew last year from an international nuclear deal signed with Tehran in 2015, and it is ratcheting up sanctions in efforts to shut down Iran's economy by ending its international sales of crude oil.

US President Donald Trump 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.

Trump said on Monday he was hopeful Iran would come to negotiating table to reach a new deal: "I really believe that Iran would like to make a deal ... and I think that's a possibility to happen."

Khamenei was quoted as saying on his website: "We said before that we will not negotiate with America, because negotiation has no benefit and carries harm."

Khamenei said Iran had no problems negotiating with Europeans and other countries, but added, "We will not negotiate over the core values of the revolution. We will not negotiate over our military capabilities."

Earlier in the day, Rouhani had taken a more positive stance.

In remarks carried by state television, he said: "Whenever they lift the unjust sanctions and fulfil their commitments and return to the negotiations table, which they left themselves, the door is not closed."

"But our people judge you by your actions, not your words."

Khamenei has the final say in all major policies under Iran's dual system, split between the clerical establishment and the government. He is also the head of the armed forces.

Iranian Foreign Ministry spokesman Abbas Mousavi said on Tuesday that Iran saw no prospect of negotiations with the United States.

Last week the Pentagon announced the deployment of 900 additional troops to the Middle East, and extended the deployment of another 600 service members in the region, describing it as an effort to bolster defences against Iran.

Speaking with reporters en route to Indonesia on Wednesday, acting US Defense Secretary Patrick Shanahan said the additional troops announced last week would be going to Saudi Arabia and Qatar. Without giving details or evidence, Shanahan said that while the Iranian posture had changed recently, the threat remained.

He added that sending military assets into the region, such as deploying bombers, Patriot missiles and accelerating the movement of an aircraft carrier strike group to the Middle East, had helped deter attacks against Americans in Iraq.

US national security adviser John Bolton also said on Wednesday naval mines "almost certainly from Iran" were used to attack oil tankers off the United Arab Emirates this month, and warned Tehran against conducting new operations.

Mousavi dismissed Bolton's remarks as a "ludicrous claim".