Trump blames Iran for Middle East instability and warns of 'massive maximum pressure'

US President says he will order end to sanctions on Syria, one of many regional countries affected by Tehran's influence

US President Donald Trump addresses the Saudi-US Investment Forum in Riyadh on May 13. EPA

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US President Donald Trump on Tuesday blamed Iran for instability in the Middle East, telling Tehran that his administration would inflict "massive maximum pressure" if it continues to attack its neighbours and support terrorism.

Mr Trump, addressing the Saudi-US Investment Forum in Riyadh on the second day of his official trip to the Gulf, accused Iran of causing "unthinkable suffering" in Syria, Lebanon, Gaza, Iraq, Yemen and beyond.

“We'll never allow America and its allies to be threatened with terrorism or nuclear attack. The choice is theirs to make," he said.

Talks between Washington and Tehran continue over a new deal to put limits on Iran's nuclear programme. Iran described the latest round of talks held in Oman as "difficult but useful". The US, meanwhile, has continued to hit Iran with sanctions.

On Tuesday, the US Treasury Department announced new sanctions on more than 20 companies in a network that it said has long sent Iranian oil to China. The network enabled the shipment of oil worth billions of dollars to China, the Treasury said in a press release.

Oscillating between a conciliatory tone and stark warnings, Mr Trump said he wants to offer Tehran a path to a "far better and more hopeful future", but would hit its oil exports if talks fail.

"If I can make a deal with Iran, I'll be very happy if we're going to make your region and the world a safer place,“ he said. "But they cannot have a nuclear weapon. This is an offer that will not last forever."

Mr Trump has said in the past that "there will bombing" if Iran and the US fail to come to a deal.

During his speech, Mr Trump announced he would lift all US sanctions against Syria to allow the new government "a chance at greatness".

"It's their time to shine. We're taking them all off," he said, adding that his administration would move towards normalising relations with Syria's new government and that Secretary of State Marco Rubio would meet Syrian Foreign Minister Asaad Al Shibani in Turkey this week.

He said "countless lives were lost in the Iranian effort to maintain a crumbling regime in Syria", referring to the government of former president Bashar Al Assad, which waged a brutal civil war against opposition groups for a decade before being toppled last year.

Iran invested billions of dollars to prop up the Assad regime and Iranian advisers helped to protect the government in Damascus during the civil war.

Mr Al Assad's successor, former militant leader Ahmad Al Shara, has been seeking to mend ties with Washington after more than a decade of war.

A White House official said that Mr Trump will "say hello" to Mr Al Shara during his visit to Saudi Arabia.