Joe Biden attends the dignified transfer of the remains of three US service members, who were killed in Jordan during a drone attack, at Dover Air Force Base in Delaware, US, on Friday. Reuters

The US military launched retaliatory air strikes against Iranian forces and Tehran-backed militia groups in Iraq and Syria on Friday, killing at least 18 fighters, following a deadly drone attack on an American base in Jordan.

The United States blamed the Sunday drone attack on Iran-backed forces, but did not strike the country's territory, with Washington seeking to deter future attacks while stopping short of all-out war with Tehran —something both sides have sought to avoid.

"Our response began today. It will continue at times and places of our choosing," President Joe Biden said in a statement.

"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," he added.

The strikes targeted the Islamic Revolutionary Guard Corps Quds Force as well as "affiliated militia groups," with American forces including long-range bombers flown from the United States hitting "more than 85 targets," the US Central Command (CENTCOM) said in a statement.

"The airstrikes employed more than 125 precision munitions," CENTCOM said. Targets included command and control and intelligence centers as well as rocket, missile and drone storage facilities belonging to militia groups and Iranian forces "who facilitated attacks against US and coalition forces."

The strikes killed at least 18 pro-Iran fighters, according to the Syrian Observatory for Human Rights war monitor.

Members of the US Honour Guard carry the remains of three US service members who were killed in Jordan. Reuters

US National Security Council spokesman John Kirby told journalists the strikes lasted about 30 minutes, though they involved a lengthy trip for the B-1 bombers that flew from the United States.

He said the Defense Department is still assessing damage from the strikes -- which hit targets at seven separate facilities — but that the United States believes the raids were successful, and made clear that more would follow.

Syrian Observatory for Human Rights chief Rami Abdel Rahman said that at least 26 major sites housing pro-Iranian groups were destroyed in Syria, including weapons depots.

A weapons warehouse and a command center belonging to pro-Iranian groups were also targeted in western Iraq, along the Syrian border, two Iraq security sources told AFP, resulting in at least "some injuries."

The strikes represent a "significant escalation," according to Allison McManus, managing director for national security and international policy at the Center for American Progress.

But she was skeptical about the impact, adding: "We have not seen that similar tit-for-tat strikes have had a deterrent effect."

Agence France-Presse