The United States has launched airstrikes in Iraq and Syria against more than 85 targets linked to Iran's Revolutionary Guard (IRGC) and the militias it backs, reportedly killing more than 30 people, in retaliation for a deadly attack on US troops.
The strikes, which included the use of long-range B-1 bombers flown from the United States, were the first in a multi-tiered response by President Joe Biden's administration to the attack last weekend by Iran-backed militants.
More US military operations were expected in the coming days.
The strikes intensified a conflict that has spread into the region since war erupted between Israel and Hamas after the militant Palestinian group's deadly assault on Israel on October.7.
Iran's foreign ministry spokesperson Nasser Kanaani said in a statement the US attacks represented "another adventurous and strategic mistake by the United States that will result only in increased tension in instability in the region".
Iraq also condemned the US attacks, saying they had killed 16 people including civilians. In Syria, the strikes killed 23 people who had been guarding the targeted locations, said Rami Abdulrahman, director of the Syrian Observatory for Human Rights, an organisation that reports on war in Syria.
US Lieutenant General Douglas Sims, the director of the Joint Staff, said the attacks appeared to be successful, triggering large secondary explosions as the bombs hit militant weaponry.
He said the strikes were undertaken knowing that there would likely be casualties among those in the facilities.
Despite the strikes, the Pentagon has said it does not want war with Iran and does not believe Tehran wants war either, even as Republican pressure has increased on the Biden to deal a blow directly.
Iran, a backer of Hamas, has sought to stay out of the regional conflict itself even as it supports groups that have entered the fray from Lebanon, Yemen, Iraq and Syria - the so-called "Axis of Resistance" that is hostile to Israeli and US interests.
US Defense Secretary Lloyd Austin said after the strikes that Biden had directed additional action against the IRGC and those linked to it.
"This is the start of our response," Austin said.
"We do not seek conflict in the Middle East or anywhere else, but the president and I will not tolerate attacks on American forces."
An Iraqi government statement said the areas bombed by US aircraft included places where Iraqi security forces are stationed near civilian locations.
It said 23 people had been wounded as well as the 16 killed.
The White House said the United States had informed Iraq ahead of strikes.
Baghdad later accused the United States of deception, saying a US claim of coordination with the Iraqi authorities was "unfounded".
The Syrian foreign ministry said the United States was fuelling conflict in the region in a "very dangerous way".
On Friday, Iran's President Ebrahim Raisi said his country would not start a war, but it would "respond strongly" to anyone who bullied it.
Hamas condemned the US strikes and said Washington was pouring "oil on the fire".
Britain called the United States its "steadfast" ally and said it supported Washington's right to respond to attacks.
The strikes hit targets including command and control centers, rockets, missiles and drone storage facilities, as well as logistics and munition supply chain facilities, the US military said in a statement.
In Iraq, local residents said several strikes hit the Sikak Neighborhood in Al-Qaim, a residential area that locals said was also used by armed groups to store large amounts of weapons.
US troops have been attacked more than 160 times in Iraq, Syria and Jordan since October 7, usually with a mix of rockets and one-way attack drones, prompting the United States to mount several retaliatory attacks even before the latest strikes.
"Our response began today. It will continue at times and places of our choosing," Biden said.
The top Republican on the Senate armed services committee, Roger Wicker, criticised Biden for failing to impose a high enough cost on Iran, and taking too long to respond.
Iranian advisers assist armed groups in both Iraq, where the United States has about 2500 troops, and Syria, where it has 900.
