The US has hit radar sites in Yemen after one of its warships in the Red Sea came under missile attack for the second time in days.
The Pentagon said the sites were on territory controlled by Houthi rebels. The rebels denied firing the missiles.
It marks the first time the US has fired at rebel targets since the start of the Yemen conflict in March 2015.
The US said initial assessments showed three radar sites involved in the recent attacks had been destroyed.
It said the US strikes had been authorised by President Barack Obama.
They were carried out using Tomahawk cruise missiles fired from the destroyer USS Nitze, according to US officials.
"These limited self-defence strikes were conducted to protect our personnel, our ships, and our freedom of navigation in this important maritime passageway," Pentagon spokesman Peter Cook said.
"The United States will respond to any further threat to our ships and commercial traffic, as appropriate."
The Houthi-controlled Saba news agency quoted an unnamed rebel official as saying neither the movement nor its allies were involved in any of the recent missile attacks.
The group has been fighting pro-government forces and a Saudi-led coalition since March 2015, after forcing the Yemeni president and his government into exile.
A real enough threat: Jonathan Marcus, BBC defence and diplomatic correspondent
Two attacks against a US naval vessel within the past four days prompted the cruise missile strikes against radar sites located in what the Americans say is Houthi-controlled territory.
An earlier attack on a UAE-leased high-speed transport vessel caused significant damage - so the threat from these land-based missiles - thought to be a variant of the Chinese C-802 - is real enough.
Quite why the Houthi rebels decided to attack the US ships now is unclear. The US of course, though troubled by several aspects of the Saudi-led air campaign against the Houthis, remains a close ally of Saudi Arabia.
The Obama administration clearly took the view that three missile attacks on shipping could not go unanswered. A direct strike against presumed Houthi positions marks the first direct US involvement in the conflict and many in Washington will hope that this will be its last.
The US attack came hours after at least one missile was fired at the USS Mason, a guided-missile destroyer, in the Red Sea off Yemen.
The Pentagon said the ship took defensive action and suffered no damage.
On Sunday, two missiles were fired at the USS Mason from the same rebel-held territory.
Both missiles hit the water before reaching the ship, the Pentagon said.
At the time, a Houthi spokesman told the Saba news agency that it had not targeted any warships.
A Saudi-led multinational coalition, supported by the US, is carrying out an air campaign against the Houthi movement.
However, US support for the coalition has come under strain following an air strike on a funeral hall in the capital Sanaa earlier this month that killed at least 140 people.
Washington said it would review its support to "better align with US principles, values and interests".
The Saudi government has not publicly acknowledged that its planes carried out the strike, but it has launched an inquiry.
It has also said it will facilitate the evacuation of Yemenis injured in the attack who need medical treatment abroad.
The UN says at least 4,125 civilians have been killed and 7,207 injured since the coalition intervened in the conflict between forces loyal to Yemen's internationally-recognised government and those allied to the Houthis in March 2015.