President Joe Biden has said that a US air strike against an Iranian-backed militia in eastern Syria, the first since he took office, should be seen by Iran as a warning.
Asked what the message was from the air strike, Biden said: "You can't act with impunity."
"Be careful," he added, speaking in Houston during a tour of relief efforts after a huge winter storm in Texas.
Syria and Iran on Friday condemned the attack with Damascus calling it a "bad sign" from the new Biden administration and Tehran saying it would further destabilize the region.
Pentagon spokesman John Kirby said two F-15E "Strike Eagles" dropped seven precision-guided munitions on Thursday on facilities in eastern Syria used by the militias believed to be behind a spate of rocket attacks on US troops in Iraq.
Secretary of State Antony Blinken said the administration wanted to make it "very, very clear -- notably to Iran-- that they cannot act with impunity against our people, our partners, our interests.
"And I think -- and expect -- that that message was clearly received," Blinken said.
Syria condemned the strike as "cowardly American aggression."
The Iranian foreign ministry strongly condemned what it called "illegal attacks" that are a "clear violation of human rights and international law."
"America's recent action strengthens and expands the activities of the terrorist Daesh (Islamic State) in the region," Shamkhani, the Secretary of Iran's Supreme National Security council, said in remarks to visiting Iraqi Foreign Minister Fuad Hussein.
"The attack on anti-terrorist resistance forces is the beginning of a new round of organized terrorism," the semi-official Nour News quoted him as saying.
The Syrian Observatory for Human Rights said at least 22 fighters from Iraq's state-sponsored Hashed al-Shaabi paramilitary force were killed.
The US action followed three rocket attacks on facilities in Iraq used by US and coalition forces fighting IS.
Last week, the Biden administration offered talks with Iran led by European allies as it seeks to salvage a 2015 nuclear deal. But the new administration has also made clear it would not brook "malign activities" by Iran.
Iran is believed to be searching for an opportunity to avenge the US assassination of top general Qasem Soleimani one year ago. Soleimani, a senior Revolutionary Guards commander, was Iran's key liaison to its allies in Iraq and Syria, and elsewhere in the region.
He was killed in a US drone strike just as he arrived in Baghdad for meetings with top Iraqi officials.