The United States fired a barrage of cruise missiles into Syria early Friday in retaliation for Tuesday's chemical weapons attack in Idlib, which killed over 70 civilians, US officials said.
Regional allies including Turkey welcomed the intervention as a positive move.
Fifty-nine US Tomahawk missiles, fired from warships in the Mediterranean Sea, targeted Shayrat air base in retaliation for the attack that American officials believe Syrian regime aircraft carried out using a nerve agent, possibly sarin.
Syria denies that it uses chemical weapons. Regime backer Russia on Friday said it wants an emergency meeting of the UN Security Council to discuss the missile strikes, which the Russian foreign ministry described as "thoughtless."
"This is not the first time the United States has resorted to such a thoughtless step, which merely exacerbates existing problems and threatens global security," the foreign ministry said in a statement.
The cruise missile strikes were the first direct American assault on the Syrian regime and US President Donald Trump's most dramatic military order since becoming president.
"It is in the vital national security interest of the United States to prevent and deter the spread and use of deadly chemical weapons," Trump said following the military strike.
"There can be no dispute that Syria used banned chemical weapons, violated its obligations under the chemical weapons convention and ignored the urging of the UN Security Council."
The missiles were fired from two US destroyers in the eastern Mediterranean and hit the air base at approximately 3:45 am (Syrian local time) on Friday (0045GMT), the Pentagon said on Friday.
Aircraft, hardened aircraft shelters, petroleum and logistical storage, ammunition supply bunkers, air defence systems, and radars were targeted, it said.
Syrian regime media said that four children among nine civilians were killed in the strike.
The assault was a reversal for Trump, who warned when campaigning for the presidency against the US getting pulled into the Syrian war, now in its seventh year.
But Trump appeared moved by the photos of children killed in the chemical attack, calling it a "disgrace to humanity" that crossed "a lot of lines.
US Navy Captain Jeff Davis said that Russian forces were notified in advance of the strike "using the established deconfliction line."
"US military planners took precautions to minimise risk to Russian or Syrian personnel located at the airfield."
Russia and Syrian regime react
The president did not announce the attacks in advance, though he and other national security officials ratcheted up their warnings to the Syrian regime throughout the day on Thursday.
"I think what happened in Syria is one of the truly egregious crimes and shouldn't have happened and it shouldn't be allowed to happen," Trump said.
Russian President Vladimir Putin, a staunch ally of Syrian regime leader Bashar al Assad, called the US action as "aggression against a sovereign nation" on a "made-up pretext" and as a cynical attempt to distract the world from civilian deaths in Iraq, according to Putin's spokesman Dmitry Peskov.
This step by Washington inflicts considerable damage to US-Russia relations, which are already in a lamentable state — Dmitry Peskov
At least four Syrian soldiers, including a senior officer, were killed in the attack, which almost completely destroyed the base, the Britain-based Syrian Observatory for Human Rights said on Friday.
The US strikes "targeted military positions in Syria and in Homs specifically" in order to publicly "serve the goals of terrorism in Syria and the goals of Israel in the long run," said the governor of the Syrian city of Homs, Talal Barazi.
Barazi said that the "Syrian policy will not change" and the US attack was a form of "support for the armed terrorist groups, and it is an attempt to weaken the capabilities of the Syrian Arab Army to combat terrorism".
Prior to the military strike, Russia warned the US on Thursday that there could be "negative consequences" if Washington takes military action against Syria.
"All responsibility if military action occurs will be on the shoulders of those who initiated such a doubtful tragic enterprise," Russian Ambassador Vladimir Safronkov told reporters following a closed-door UN Security Council meeting on Syria.
Russia has provided military support for the Syrian regime since September 2015, turning the balance of power in the regime's favour. Moscow has used its veto power at the UN Security Council on several occasions since the civil war began six years ago to prevent sanctions against Damascus.
Earlier on Thursday, the US Secretary of State Rex Tillerson warned that Washington would make an "appropriate response" to Tuesday's chemical attack in Syria.
Britain, France and the US have drafted a UN resolution that would demand the Syrian regime provide information on its flight operations as part of an Organisation for the Prohibition of Chemical Weapons (OPCW) probe into the incident.
The regime, however, blamed the chemical attack on Daesh and the opposition who "continue to store chemical weapons in urban and residential areas."