WASHINGTON, April 13 - U.S., British and French forces hammered Syria with air strikes early Saturday Syria time in response to a poison gas attack that killed dozens of people last week, in the biggest intervention by Western powers in Syria's civil war.U.S. President Donald Trump announced the military action from the White House late on Friday. As he spoke, explosions rocked Damascus.Trump said he was prepared to sustain the response until the government of Syrian President Bashar Assad stopped its use of chemical weapons.The intervention was the biggest strike by Western powers against Assad in the country’s seven-year-old civil war and pitted the United States and its allies against Russia, which itself intervened in the war in 2015 to back Assad."A short time ago, I ordered the United States Armed Forces to launch precision strikes on targets associated with the chemical weapons capabilities of Syrian dictator Bashar Assad," Trump said in a televised address from the White House.A U.S. official told Reuters the strikes were aimed at multiple targets and involved Tomahawk cruise missiles.At least six loud explosions were heard in Damascus in the early hours of Saturday and smoke was seen rising over the Syrian capital, a Reuters witness said. A second witness said the Barzah district of Damascus had been hit in the strikes. Barzah is the location of a major Syrian scientific research center.Speaking of Assad and his suspected role in last week's chemical weapons attack, Trump said, "These are not the actions of a man. They are crimes of a monster instead.""The purpose of our actions tonight is to establish a strong deterrent against the production, spread and use of chemical weapons," Trump said.The U.S. president had sharply critical words for both Russia and Iran, which have backed Assad's government."To Iran and to Russia, I ask, what kind of a nation wants to be associated with the mass murder of innocent men, women and children?" Trump said.British Prime Minister Theresa May said she had authorized British armed forces "to conduct co-ordinated and targeted strikes to degrade the Syrian regime's chemical weapons capability." She described it as a "limited and targeted strike" aimed at minimizing civilian casualties.The military action is not about intervening in Syria's civil war or changing its government, she said.