WASHINGTON — The United States and European allies launched strikes against Syrian targets, President Trump announced on Friday, seeking to punish President Bashar al-Assad for a suspected chemical attack near Damascus last weekend that killed more than 40 people.
Mr. Trump said Britain and France had joined the United States in the strikes, which he said were underway.
“These are not the actions of a man. They are crimes of a monster instead,” Mr. Trump, in a televised nighttime address from the White House Diplomatic Room, said of the chemical attacks that he blamed on Mr. Assad.
He said the allied strikes of precision weapons sought to deter the production, spread and use of chemical weapons as “a vital national security interest of the United States.”
“We are prepared to sustain this response until the Syrian regime stops its use of prohibited chemical agents,” Mr. Trump said.
The strikes risked pulling the United States deeper into the complex, multi-sided war in Syria from which Mr. Trump only last week said he wanted to withdraw. They also raised the possibility of confrontation with Russia and Iran, both of which have military forces in Syria to support Mr. Assad.
In London, Prime Minister Theresa May of Britain said the allied strikes on the Syrian government’s chemical weapons capability aimed not only to protect innocent civilians, but also to prevent such attacks from becoming an international norm.
“We have sought to use every possible diplomatic channel to achieve this,” Mrs. May said in a statement.
In choosing to strike, it appeared that Mr. Trump’s desire to punish Mr. Assad for what he called a “barbaric act” — and make good on his tweets promising action this week — outweighed his desire to limit the United States’ military involvement in the conflict, at least in the short term.
The strikes marked the second time that Mr. Trump has attacked Syria to punish the government after it was accused of using chemical weapons. The White House had sought to craft a response that would be more robust than the attack last April, when the United States fired 59 Tomahawk cruise missiles at a Syrian air base that was back in use a day later.
France and Britain joined the United States in planning the latest fusillade of missiles against Mr. Assad’s government, presenting an allied condemnation of the April 7 suspected chemical attack in the town of Douma. Germany, however, refused to take part in the coordinated military action in Syria, even though Chancellor Angela Merkel called the use of chemical weapons “unacceptable.”
The missiles struck Syria shortly after 4 a.m. local time on Saturday. A fact-finding mission from the Organization for the Prohibition of Chemical Weapons was to begin investigating the incident on Saturday in Douma, which had been held by rebels before the suspected attack. The mission’s job was only to determine whether chemical weapons had been used, not who had used them.
Medical and rescue groups have reported that the Syrian military dropped bombs that released chemical substances during an offensive to take the town. A New York Times review of videos of the attack’s aftermath, and interviews with residents and medical workers, suggested that Syrian government helicopters dropped canisters giving off some sort of chemical compound that suffocated at least 43 people.
On Friday, American officials said they had intelligence implicating the Syrian government.
“We have a very high confidence that Syria was responsible,” said Sarah Huckabee Sanders, the White House press secretary. She said Russia was “part of the problem” for failing to prevent the use of such weapons.
At the United Nations, Nikki R. Haley, the American ambassador to the world body, accused the Syrian government of using banned chemical arms at least 50 times since the country’s civil war began in 2011. State Department officials said the United States was still trying to identify the chemical used on April 7.
President Emmanuel Macron of France on Thursday cited proof that the Syrian government had launched chlorine gas attacks. The same day, the British Cabinet authorized Mrs. May to join the United States and France in planning strikes against Syria.
Leaders in Syria, Iran and Russia denied that government forces had used chemical weapons, and accused rescue workers and the rebels who had controlled Douma of fabricating the videos to win international sympathy.
On Friday, Maj. Gen. Igor Konashenkov, a spokesman for the Russian Defense Ministry, said images of victims of the purported attack had been staged with “Britain’s direct involvement.” He provided no evidence.
Karen Pierce, Britain’s ambassador to the United Nations, called those allegations “bizarre” and “a blatant lie.”
In the immediate aftermath of the suspected attack, Mr. Trump called Mr. Assad an “animal,” warning the Syrian leader and his Russian and Iranian backers that they would have a “big price to pay.” That suggested the United States might take action against Syria’s patrons as well.
“If it’s Russia, if it’s Syria, if it’s Iran, if it’s all of them together, we’ll figure it out and we’ll know the answers quite soon,” Mr. Trump said early in the week. “So we’re looking at that very strongly and very seriously.”
Mr. Trump’s defense secretary, Jim Mattis, had sought to slow down the march to military action as allies compiled evidence of Mr. Assad’s role that would assure the world the strikes were warranted. Mr. Mattis also raised concerns that a concerted bombing campaign could escalate into a wider conflict between Russia, Iran and the West.
Before the strikes, the United States had mostly stopped aiding Syria’s rebels, like those who were in Douma, who want to topple Mr. Assad’s government. The Pentagon’s most recent efforts in Syria have focused on the fight against Islamic State militants in the country’s east, where it has partnered with a Kurdish-led militia to battle the jihadists.
It is the roughly 2,000 American troops there that Mr. Trump said he wants to bring home.
Russian forces and Iranian-backed militias also are deployed around Syria to help fight the rebellion — including the Islamic State and other extremist groups — that has surged against Mr. Assad since the conflict started more than seven years ago.
A previous American attack on Syria, last April, came after a chemical attack on the village of Khan Sheikhoun killed scores of people. Mr. Trump ordered a cruise missile strike against the Al Shayrat airfield in central Syria, where the attack had originated. The base was damaged but Syrian warplanes were again taking off from there a day later.
Still, the response set Mr. Trump apart from President Barack Obama, who declined to respond with military force after a chemical weapons attack in August 2013 killed hundreds of people near Damascus, even though Mr. Obama had earlier declared the use of such weapons a “red line.”
Mr. Obama ultimately backed off a military strike and reached an agreement with Russia to remove Syria’s chemical weapons arsenal. That agreement was said to have been carried out, although a series of reported chemical attacks since have raised doubts about its effectiveness.
Both American presidents have sought to keep United States involvement in Syria focused on the battle against the Islamic State, and not on toppling Mr. Assad or protecting civilians from violence.
The question now for Mr. Trump is whether his intervention against Mr. Assad will make it harder to keep the United States from slipping deeper into the Syrian war.