A US official, speaking to AFP on condition of anonymity, said Iran is behind an attack on Saudi oil facilities.
Washington claims the assault involved cruise missiles and that evidence would be presented at the UN General Assembly next week.
US Vice President Mike Pence announced that Secretary of State Mike Pompeo was on his way to Saudi Arabia to “discuss our response.”
“As the president said, we don’t want war with anybody but the United States is prepared,” Vice President Pence said in a speech to the Heritage Foundation think tank in Washington.
“We’re locked and loaded and we’re ready to defend our interests and allies in the region, make no mistake about it,” he said, echoing President Donald Trump’s words on Monday.
The apparent hardening of the US position came as Iran’s supreme leader ruled out negotiations with Washington “at any level.”
However, continuing days of mixed messaging from the White House, spokesman Hogan Gidley refused to rule out a possible, albeit now highly unlikely, encounter between Trump and Iranian President Hassan Rouhani at the United Nations next week.
President Trump is “obligated to try and find peace,” he said.
Iran-Saudi proxy war
Yemen’s Iranian-backed Huthi rebels claimed responsibility for Saturday’s oil installation attacks, which halved output from the petro-state, the world’s biggest crude producer and a close ally of Washington.
The Huthis are at war with Saudi-backed forces in Yemen, turning the impoverished nation into a proxy battlefield for bitter regional rivals Tehran and Riyadh.
The increasingly complex conflict dovetails with the Trump administration’s attempt to curb Iranian power through a “maximum pressure” campaign of crippling economic sanctions.
President Trump began that campaign after unilaterally pulling out of a 2015 international deal meant to reward Iran for allowing restrictions on its nuclear industry. He says that Tehran is secretly cheating and trying to build nuclear weapons and must be stopped.
Supreme leader Ayatollah Ali Khamenei said Washington’s real goal was to bring his country to its knees.
“The policy of ‘maximum pressure’ against the Iranian nation is worthless and all Islamic Republic of Iran officials unanimously believe there will be no negotiations with the US at any level,” he said in a televised address.
Mr Khamenei also said that as long as sanctions are in place, direct talks are impossible, including a meeting at the UN.
“Without this, no negotiations will happen at any level between the Islamic Republic of Iran’s officials and the Americans, not during the visit to New York or any other visit,” he said.
Support for war?
Whether the new stage of the long-running US-Iranian standoff leads to a shooting war is the big unknown.
Saudi Arabia, which has bought huge quantities of US weaponry, is considered a strategic ally in the region, second to Israel, another bitter foe of Iran.
President Trump called off a retaliatory US missile attack on Iran in June after the Iranians shot down a spy drone. He said he did not want to kill what generals told him could be up to 150 people.
He has indicated that he may no longer feel that restraint.
“I’m not looking to get into new conflict, but sometimes you have to,” he said Monday. “That was a very large attack, and it could be met by an attack many, many times larger.”
US Secretary of Defense Mark Esper labelled Iran a destabilising force in the region, but stopped short of directly accusing Tehran over the strikes.
The US military, he said, was working with its partners to “address this unprecedented attack.”
In Congress, Republican Senator Mitt Romney told reporters that the United States should not be drawn into war.
“Saudi Arabia is fully capable, with the weapons we’ve sold them, to respond in a way they think is appropriate,” he said.
Another Republican, Senator John Thune, said that if Iran were proven to be behind the Saudi attack, that “crosses a line that hasn’t been crossed before and, yeah, I think it’d be certainly appropriate for the US and allies to respond.”
Oil market nervous
The contested strikes hit Abqaiq, the world’s largest oil processing facility, and the Khurais oil field in eastern Saudi Arabia.
Oil markets continued to eye the situation carefully. Prices receded five percent on Tuesday, reversing some of the previous day’s gains as analysts said they were expecting Saudi output to recover sooner than expected after the weekend’s attacks.’
As aftershocks continued to ripple across financial markets on Tuesday, energy specialists S&P Platt said around three million barrels per day of Saudi crude would remain offline for at least a month.
Britain and Germany on Tuesday urged the international community to forge a “collective response” to the attacks.
French Foreign Minister Jean-Yves Le Drian, visiting Cairo, said it was “imperative to put all our efforts together to achieve this de-escalation”.
China condemned the attacks and appealed to all sides to “refrain from taking actions that lead to an escalation of tensions in the region.”