Donald Trump has announced by tweet that he is increasing sanctions on Iran, following the weekend attacks on Saudi oil installations.

Giving no explanation of what the order would entail, the president tweeted: “I have just instructed the Secretary of the Treasury to substantially increase Sanctions on the country of Iran!” Mr Trump’s missive comes after repeated assertions from US officials that Iran was behind the attack.

Tehran has denied any involvement in the attacks on two Saudi oil installations, which the Iran-backed Houthi rebel group in Yemen has claimed responsibility for.

“They want to impose maximum ... pressure on Iran through slander,” Iran’s President Hassan Rouhani said. “We don’t want conflict in the region ... Who started the conflict?” he added, blaming Washington and its Gulf allies for the war in Yemen.

It was not immediately clear what new sanctions were being implemented. The US has increased sanctions on Iran since unilaterally withdrawing last year from a deal signed with other world powers to reduce Tehran’s nuclear capacity.

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Saudi Defence Ministry held a news conference on Wednesday displaying drone and missile debris it claimed is ”undeniable” evidence of Iranian aggression. A total of 25 drones and missiles were used in the attacks launched from Iran not Yemen, the ministry spokesman added.

President Trump has previously said that US and Saudi investigators were trying to determine whether Iran was responsible.

On Monday he said: “Well, it’s looking that way. We’ll have some pretty good - we’re having some very strong studies done, but it’s certainly looking that way at this moment. And we’ll let you know. As soon as we find out definitively, we’ll let you know. But it does look that way.”

Asked about whether military action was possible, he insisted that he did not want to go to war but added: “I don’t want war with anybody. But we’re prepared more than anybody.”

Iran responded by saying it would retaliate “immediately” to any attack.

Responding to news of the latest sanctions, Republican Senator Lindsey Graham told CNN that the attack on Saudi oil platforms was an “act of war” and said Iran would “pay a price”. He said: ““This attack on the oil refinery by any reasonable definition is an act of war. It is attacking the world economy.

“In addition to attacking the refinery in Saudi Arabia, the Iranians are increasing enrichment, putting them closer to a bomb.”

Three US officials told Reuters the attack was launched from Iran’s southwest, and that it involved both cruise missiles and drones. The officials did not provide evidence or detail the US intelligence they used to make the assessments.

US secretary of state, Mike Pompeo is due to meet Saudi Arabia’s Crown Prince Mohammed bin Salman in Jeddah on Wednesday to discuss the crisis, before heading to the United Arab Emirates.

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There had been suggestions that Mr Trump could use this month’s United Nations general assembly meeting in New York as an opportunity to meet his Iranian counterpart, Mr Rouhani, although this now looks extremely unlikely.

There were even questions as to whether Iran’s delegation would be given visas allowing them to attend the meeting because of US sanctions.

Foreign minister Javad Zarif was due to travel to New York on Friday, with Mr Rouhani following on Monday, according to the IRNA news agency.

As the host of the UN’s headquarters, the US is mandated to offer world leaders and diplomats visas to attend meetings there. But as tensions have risen, greater restrictions have been imposed on officials like Mr Zarif. Since becoming president in 2013, Mr Rouhani has spoken each year at the general assembly.