"Last night's attack on Syria and Iraq is an adventurous action and another strategic mistake by the US government, which will have no result other than intensifying tensions and instability in the region," Iran's foreign ministry spokesman Nasser Kanani said in a statement.

"The US military attacks on Iraq, Syria and Yemen merely provide for the goals of the Zionist regime," he said, referring to US ally Israel.

The American strikes were "a violation of the sovereignty and territorial integrity of Iraq and Syria, of international law and a flagrant violation of the United Nations charter", Kanani added.

The United States launched air strikes against Iran-allied militias in Iraq and Syria on Friday, with President Joe Biden vowing more to come, in retaliation for a drone attack that killed three US soldiers in Jordan.

The US military said it attacked a total of 85 targets at seven different sites in Syria and Iraq.

The strikes killed at least 18 pro-Iranian fighters in Syria, said the Syrian Observatory for Human Rights war monitor. In Iraq, they killed 16 people, including civilians, the Baghdad government said.

The US military has also been carrying out air strikes on Yemen's Iran-backed Huthi rebels over attacks on international shipping.

The Huthis began targeting Red Sea shipping in November, saying they were hitting Israel-linked vessels as a way to support Palestinians in Gaza, which has been ravaged by the Israel-Hamas war.