Members of the US Congress blamed Iran after an attack on Saudi Arabian crude oil facilities on Sunday but said the US should be careful about any retaliatory military action — especially because a clearer picture of who was behind the attack was needed.
President Donald Trump has said it is "looking like" Iran was behind the attacks, despite a claim from Houthi rebels that they launched the attack from Yemen, but insisted that he did not want war.
He said the US was prepared if for retaliation if it was warranted but would need "definitive proof" that Tehran was responsible.
The strikes knocked out more than 5% of global oil supply.
President Donald Trump tweeted on Monday: "There is reason to believe that we know the culprit," adding that the US was is "locked and loaded" and awaiting information from Saudi Arabia.
Senior US officials told reporters that damage from the attacks indicated they came from the direction of Iraq and Iran, not from Yemen.
Yemen's vice president Ali Mohsen Al-Ahmar condemned the attacks on the oil facilities in a tweet, saying they revealed Iran's "destructive role in the region" and its use of Yemen to stage operations.
"We condemn this blatant assault on economic security and stand with our brothers in the kingdom to deter Iran's malicious arms," said Ahmar.
But Iran has denied the attack and said it was ready for "full-fledged war".
NATO chief Jens Stoltenberg told AFP on Monday he was "extremely concerned" about the escalating tensions and accused Iran of destabilising the region.
"We call on all parties to prevent any such attacks occurring again because that can have negative consequences for the whole region, and we are also extremely concerned about a risk of escalation," he said.