WASHINGTON – President Donald Trump said Wednesday that he would slap new sanctions on Iran but played coy when asked if he would retaliate more aggressively for what his secretary of state called "an act of war" against Saudi Arabian oil facilities.

"I have just instructed the Secretary of the Treasury to substantially increase Sanctions on the country of Iran!" Trump tweeted Wednesday morning. He didn't offer specifics.

Trump said later that he would make another announcement on the sanctions within 48 hours.

“If we have to do something, we’ll do it without hesitation,” Trump said. “There are many options. There’s the ultimate option.”

Secretary of State Mike Pompeo called the attack on the Saudis an "act of war" perpetrated by Iran.

"This was an Iranian attack," Pompeo said as he traveled to Saudi Arabia, where he planned to meet with the kingdom's leaders and discuss a response. He said Iran's supreme leader "put at risk the global energy supply" by orchestrating the attack.

"We’re blessed that there were no Americans killed in this attack, but any time you have an act of war of this nature, there’s always risk that that could happen," Pompeo said. "This is an attack of a scale we’ve just not seen before."

A Saudi military spokesman, Col. Turki al-Malki, said Wednesday the strikes on oil sites were "unquestionably sponsored by Iran." He stopped short of saying Tehran launched the strikes.

The Trump administration is considering a variety of steps, both economic and military, to retaliate for last weekend's attack, which temporarily slashed Saudi Arabia's oil-producing capacity.

Pompeo said he was trying to build "a coalition to develop a plan to deter" Iran from further attacks.

Iranian officials denied any role in the attack and accused the United States of warmongering. Tehran warned of an "immediate response" if the United States strikes Iran.

The Houthis, an Iranian-backed rebel group at war with Saudi Arabia, claimed responsibility for the strikes.

Pompeo called the Houthis' claim "fraudulent."

"The intelligence community has high confidence that these were not weapons that would have been in possession of the Houthis," he said.

Saturday's strikes hit the Abqaiq oil processing plant and a Saudi oil field, where about 5.7 million barrels of oil are produced each day. The attacks disrupted more than 5% of the world's daily supply.

A senior administration official, who briefed reporters Tuesday on the condition of anonymity, said the Houthis could not have carried out such a sophisticated operation.

Pompeo:Secretary of State Mike Pompeo heads to Saudi Arabia to discuss Iran response

Trump talked to other world leaders about the situation, including British Prime Minister Boris Johnson.

"They condemned the attacks and discussed the need for a unified diplomatic response from international partners," Johnson's office said in a statement.

Iran is likely to be a major topic for Trump when he attends United Nations meetings next week. Trump offered a small olive branch to the Iranians on Wednesday, saying he believes a delegation from Tehran should be able to come to the USA next week for the U.N. General Assembly in New York. Iranian state media reported Wednesday that Rouhani considered canceling his trip over the delay in receiving visas.

“I would let them come,” said Trump, who has long embraced the idea of meeting with foreign adversaries, including Rouhani and North Korea’s Kim Jong Un. “I’ve always felt the United Nations is very important. … I don’t think it’s ever lived up to the potential it has.”

Trump and his advisers said they are considering a variety of aggressive actions toward Iran, ranging from more cyberattacks to a strike against Iranian oil facilities.

The Pentagon is planning possible military responses, Marine Gen. Joe Dunford, chairman of the Joint Chiefs of Staff, said Tuesday.

Dunford stressed that economic, diplomatic and political options are also under consideration.

"In the region, wherever it originated from, the most likely threat is either Iran or Iranian-backed proxies," Dunford said. "Without getting out in front of the Saudi investigation, I think that is a reasonable conclusion."

Central Command, which oversees U.S. military operations in the Middle East, sent a team to Saudi Arabia to sift through traces of the weapons used to determine their origin. The Saudis have not requested additional military resources.

Dunford noted that Iranian-backed proxies have struck Saudi Arabia several times, but the attack Saturday attack was the largest.

"Without prejudging intelligence, this looked like a very complex, precise attack, not consistent with previous Houthi attacks," Dunford said.

Sen. Lindsey Graham, R-S.C., a defense hawk and a staunch Trump ally, said new sanctions will not be enough to deter Iran from attacks.

“I am looking for a response that would be unequivocal. If they don’t pay a price for bombing a neighbor’s oil fields, then all hell is going to break out in the Mideast,” Graham said Wednesday on Capitol Hill.

He said the United States should build support for a multilateral response.

“I am not looking for a response immediately,” he said. "But I am looking for a response that would restore deterrence, and my belief is that additional sanctions will fall short.”

The Trump administration has already slapped Iran with crippling sanctions, aimed at driving the country’s oil exports to zero and choking Tehran’s economy.

The goal, administration officials said, is to force Iran back to the negotiating table after Trump withdrew from a nuclear deal last year. Trump said that deal, reached in 2015, was not restrictive enough, and he wants a new agreement that would limit Iran’s ballistic missile program and its support for terrorism in the region.

Trump has vacillated between rhetoric suggesting a military response and a softer tone emphasizing his desire to avoid war.

Critics said Trump's aggressive sanctions regime has caused the crisis, and Iran lashes out in response to what it calls "economic warfare." Democrats in Congress warned that a military strike could lead to all-out war that would cost American lives and disrupt major segments of the oil production system. They said Trump should seek congressional authorization before taking any such step.

"We need to have a conversation as a country and as a Congress about whether or not this is an attack so directly relevant to America's interests as to justify military action," Sen. Chris Coons, D- Del., told CNN.

Contributing: The Associated Press