Nearly 40 people are reported to have been killed after the US military launched airstrikes on at least 85 targets in Iraq and Syria used by Iranian-backed militias on Friday.
Numerous aircraft were used - including long-range B-1 bombers flown from the US - marking the opening salvo of retaliation for the drone strike that killed three US troops in Jordan last weekend.
Iraq's Popular Mobilisation Forces, a state security force including Iran-backed groups, said 16 of its members were killed including fighters and medics. The government earlier said at least 16 people, including civilians, were dead and 23 people were wounded.
The UK-based Syrian Observatory for Human Rights reported that 23 people had been killed in Syria, bringing the total number killed in both countries to 39.
Middle East latest:
US begins wave of airstrikes in Iraq and Syria
A statement from US Central Command said: "US Central Command (CENTCOM) forces conducted airstrikes in Iraq and Syria against Iran's Islamic Revolutionary Guards Corps (IRGC) Quds Force and affiliated militia groups.
"US military forces struck more than 85 targets, with numerous aircraft to include long-range bombers flown from United States.
"The airstrikes employed more than 125 precision munitions."
The strikes targeted "command and control operations, intelligence centres, rockets, and missiles, and unmanned aerial vehicle storages, and logistics and munition supply chain facilities".
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.
President Joe Biden and other top US leaders have been warning for days that the country would strike back at the militias, and made it clear that it would not be just one hit, but a "tiered response" over time.
Following the strikes, Mr Biden said in a statement: "Our response began today. It will continue at times and places of our choosing.
"The United States does not seek conflict in the Middle East or anywhere else in the world. But let all those who might seek to do us harm know this: If you harm an American, we will respond."
On Saturday, a spokesperson for the UK government said it "supports" America's "right to respond" to attacks.
"We have long condemned Iran's destabilising activity throughout the region, including its political, financial and military support to a number of militant groups," a statement said.
Iraq warns region 'on brink of the abyss'
In response, the Iraqi foreign ministry on Saturday summoned the US charge d'affaires in Baghdad to deliver a formal memorandum of protest.
In a statement, the Iraqi government said the strikes targeted security forces in the Akashat and al Qaim regions, as well as "neighbouring civilian places".
Syrian television has claimed the area of Deir ez Zur was a target, while other regions that were reportedly targeted included Mayadin and Bukamal in Syria, according to The New York Times.
"This aggressive strike will put security in Iraq and the region on the brink of the abyss, and it also contradicts efforts to establish the required stability," a statement by Iraqi government spokesperson, Bassem al Awadi, said.
"Iraq reiterates its refusal to let its lands be an arena for settling scores, and all parties must realise this. Our country's land and sovereignty are not the appropriate place to send messages and show force between opponents.
"The Iraqi government will make every effort required by moral, national, and constitutional responsibility to protect our land, our cities, and the lives of our children in all types of armed forces."
The leader of an influential Iraqi militia group told Sky News that it does "not accept or acquiesce to the violation of Iraqi sovereignty".
Dr Firas al Yasser from the Harakat Hezbollah al Nujaba group said: "We have said it before in a clear and direct way, we don't accept threats.
"At the moment, if any members of the Islamic Resistance in Iraq are targeted, or any jihadi or resistance fighter from any part of the resistance forces are targeted, there will be retaliation from us."
Meanwhile, the Syrian foreign ministry said what the US has committed has "served to fuel conflict in the Middle East in a very dangerous way".
Russia condemned the airstrikes and said the situation should be considered by the UN Security Council.
What we know about the sites targeted
The White House said it had informed Iraqi officials ahead of the military strikes, which the Iraqi government later claimed was not true, accusing the US of "falsifying the facts".
"The American side then deliberately deceived and falsified the facts, by announcing prior coordination to commit this aggression, which is a false claim aimed at misleading international public opinion and disavowing legal responsibility for this rejected crime in accordance with all international laws and laws," the statement said.
An Iraqi security official told NBC News the US targeted a weapons warehouse and three houses belonging to Kata'ib Hezbollah in Anbar province, western Iraq.
On Friday, Mr Biden and top defence leaders joined grieving families as the remains of the three Army Reserve soldiers were returned to the US during a ceremony at Dover Air Force Base in Delaware.
The US strikes - of which there were three in Iraq and four in Syria - appeared to stop short of directly targeting Iran or its Revolutionary Guard Quds force.
Iran has denied it was behind the Jordan attack.
Its foreign ministry spokesperson Nasser Kanaani said in a statement the attacks represented "another adventurous and strategic mistake by the United States that will result only in increased tension and instability".
Before the retaliatory strikes, Iranian President Ebrahim Raisi said that Iran will not start a war but would "respond strongly" to anyone who tried to bully it.
Meanwhile, in Gaza, Israeli airstrikes killed 18 Palestinians in the cities of Rafah and Deir al Balah, Gaza health officials said on Saturday.
A Israeli spokesperson said: "In stark contrast to Hamas' intentional attacks on Israeli men, women and children, the IDF follows international law and takes feasible precautions to mitigate civilian harm."