The White House on Monday vowed a "very consequential response" to a drone attack on a base in Jordan that killed three American troops, with President Joe Biden blaming Iran-backed militants.
They were the first US military deaths in an attack in the region since the Israel-Hamas war began.
Though the casualties raised fears of an escalating conflict, National Security Council spokesman John Kirby said Washington is not seeking a wider Middle East war.
Iran said it had nothing to do with the attack and denied US and British accusations that it supported militant groups responsible for the strike on the remote frontier base in Jordan's northeast, near the borders with Iraq and Syria.
While the United States is still gathering the facts, "we know it was carried out by radical Iran-backed militant groups operating in Syria and Iraq," Biden said Sunday, pledging to hold "all those responsible to account at a time and in a manner of our choosing."
The response will be "very consequential," National Security Council spokesman John Kirby told CNN Monday.
"But we don't seek a war with Iran. We're not looking for a wider conflict in the Middle East," he added.
Kirby would not speculate on the options being considered by the president, including whether targets inside Iran were on the table.
He said Washington wants to make clear that the attack – part of a series of other increasingly dangerous assaults by Iran-backed militants in the region in recent weeks – are "unacceptable."
British Foreign Secretary David Cameron joined Biden Sunday in blaming "Iran-aligned militia" and called on Tehran to "de-escalate the region."
Iran denied any links to the attack, with foreign ministry spokesman Nasser Kanani describing the accusations as "baseless" and a "projection."
"The Islamic Republic of Iran does not welcome the expansion of conflict in the region," said Kanani in a statement Sunday, adding that Tehran "is not involved in the decisions of the resistance groups."
With the region already tense as fighting rages in Gaza, the strike also raises fears of inflaming a conflict directly involving Tehran.
There has so far been no claim of responsibility for the strike, although on Sunday the Islamic Resistance in Iraq claimed to have launched three drone attacks at bases in Syria, including near the Jordanian border.
The group – a loose alliance of Iran-linked armed groups that oppose US support for Israel in the Gaza conflict and wants them out of Iraq – has claimed dozens of attacks on US and anti-jihadist coalition forces in Iraq.
Regional explosion
A spokesman for Iran-backed Hamas, Sami Abu Zuhri, said the Jordan attack was a message that the continuation of fighting in Gaza "risks a regional explosion."
The US Central Command (CENTCOM) said late on Sunday the attack had hit the remote Tower 22 logistics support base and that 34 personnel were also wounded, eight of whom required evacuation.
There are around 350 US Army and Air Force personnel at the base who operate in support roles, including for the international coalition against the Islamic State jihadist group, CENTCOM said.
The escalating Middle East conflict poses a challenge to Biden in an election year, and Republican politicians were quick to take aim at him over the weekend.
US and allied forces in Iraq and Syria have been targeted in more than 150 attacks since mid-October, according to the Pentagon, and Washington has carried out retaliatory strikes in both countries, including against pro-Iran militants.
The latest round of the Israel-Hamas conflict began when the Palestinian militant group carried out an unprecedented attack on October 7 that resulted in about 1,140 deaths, mostly civilians, according to an AFP tally of official figures.
Israel retaliated with a relentless military offensive that has killed at least 26,422 people in Gaza, most of them women and children, according to the territory's health ministry.
Anger over that campaign has grown across the region, with violence involving Iran-backed groups in Lebanon, Iraq and Syria, as well as Yemen.
There have been near-daily exchanges of fire between Hezbollah and Israel in Lebanon. US forces are directly involved in Iraq, Syria and Yemen.
The United States and Britain have both carried out strikes targeting Yemen's Iran-backed Huthi rebels, who have been attacking Red Sea shipping in support of Palestinians in Gaza for more than two months.
