Combo picture of U.S. President Donald Trump (L) and Iran's President Hassan Rouhani. (Xinhua)

"I have just instructed the Secretary of the Treasury to substantially increase Sanctions on the country of Iran!" Trump tweeted.

WASHINGTON, Sept. 18 (Xinhua) -- U.S. President Donald Trump said on Wednesday that he has ordered a substantial increase in sanctions against Iran.

"I have just instructed the Secretary of the Treasury to substantially increase Sanctions on the country of Iran!" he said in a morning tweet.

Trump's new move came after Saturday's drone attacks on oil production facilities in eastern Saudi Arabia, which have raised global concerns over the broader Middle East region. Washington has been indicating that Iran was behind the attacks, a claim that has been strongly denied by Tehran.

U.S. Secretary of State Mike Pompeo is currently traveling to Saudi Arabia and the United Arab Emirates to discuss the recent attacks with local authorities.

Following its unilateral exit from the 2015 Iran nuclear deal last year, Washington has been piling up pressure on Tehran through a series of sanctions in an effort to force it to return to negotiations. Iran, however, maintains a tough stance and vows to retaliate against the United States' anti-Iran moves.