At least four Syrian soldiers have been killed and an airbase almost destroyed by an American missile attack, the Syrian Observatory for Human Rights says.
Watch US Defence Department footage of today's missile strike:
It is the first direct US military action against Syria and its leader, Bashar al-Assad.
The attack against an air base in the west of the country comes in response to more than 80 civilian deaths from chemical weapons in a rebel-held town.
Look back at RNZ's live coverage as the strike came to light
The Pentagon said it fired 59 Tomahawk cruise missiles at 4.40am (about 1.40pm NZT) from destroyers in the eastern Mediterranean.
In a televised address, President Donald Trump said the base was the launch point for the chemical attack.
He called on "all civilised nations" to help end the conflict in Syria.
More than 80 civilians, including many children, died in the suspected nerve gas attack on Tuesday (late Wednesday NZT) in the town of Khan Sheikhoun in Idlib province.
The retaliatory cruise missile strike was a "one-off," an unnamed American defence official told Reuters, meaning it was expected to be a single strike with no current plans for escalation.
Russia reacts
According to reports out of the Kremlin, President Vladimir Putin believed the US attack broke international law and seriously hurt US-Russia relations, Reuters reported.
The Russian leader, who is a staunch ally of Syria's Bashar al-Assad, regarded the strike 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.
The Kremlin said the Syrians did not possess chemical weapons and the US move would inevitably create a serious obstacle to creating an international coalition to fight terrorism.
Shortly before the US missiles were launched, Russia's deputy envoy at the UN warned of "negative consequences" if the US military carried out military strikes.
Pentagon says Russia informed
The Pentagon said the Russian military, which is supporting the Syrian government, was informed ahead of the US attack.
In a statement it said missiles fired from Navy destroyers USS Porter and USS Ross had targeted aircraft, aircraft shelters, storage areas, ammunition supply bunkers, air defence systems and radars at the Shayrat airfield in western Homs province.
The Pentagon added that the strike was intended "to deter the regime from using chemical weapons again".
It did not give details of damage or casualties but the governor of Homs province said there had been deaths and parts of the base were on fire.
"It will take some time to determine the extent of the damage," Talal Barazi told AFP.
An October 2016 file photo of the Shayrat airfield released by the US Department of Defence. Photo: HO / US Department of Defense / AFP
The Syrian Observatory for Human Rights, a UK-based organisation that reports on the war, said four Syrian soldiers were killed, including a senior officer. It cited its own sources. The Syrian army could not be reached for comment, Reuters reported.
A statement on Syrian state TV said "American aggression" had targeted a Syrian military base with "a number of missiles" but gave no further details.
NZ had several hours notice
Prime Minister Bill English said New Zealand was given a "couple of hours" notice that the US was considering an air strike against Syria.
He told Checkpoint with John Campbell the UN Security Council was New Zealand's preferred body to sort out the problems, but it had been unable to act.
"We've seen the atrocities with the use of chemical weapons ... We support action that is proportionate to the requirement to stop further atrocities."
Australian Prime Minister Malcolm Turnbull strongly supported the American action, saying the retribution for the chemical gas attack was proportionate, calibrated and swift.
'Dramatic' shift in policy
The US has led a coalition carrying out air strikes against jihadist groups in Syria since 2014 but this is the first time it has targeted government forces.
"What we have seen is one of the most dramatic about-turns in foreign policy that I have ever witnessed" - BBC North America Editor Jon Sopel
The latest action was welcomed by Syrian opposition group the Syrian National Coalition.
"We hope for more strikes... that these are just the beginning," spokesman Ahmad Ramadan told AFP.
"It's fairly easy to predict that Turkey will be very pleased with these latest US strikes" - CCN Istanbul correspondent Muhammad Lila
Mr Trump had earlier warned that "something should happen" against the Syrian leadership following the deaths in Khan Sheikhoun, but gave no details.
On Thursday, US Secretary of State Rex Tillerson signalled a sudden shift in policy by the Trump administration, saying that Bashar al-Assad should have no role in a future Syria.
Only last week the US ambassador to the UN, Nikki Haley, said Washington was no longer prioritising the removal of the Syrian president.
The UN Security Council will hold further talks on Friday as it tries to agree a resolution calling for an investigation into the deaths in Khan Sheikhoun.
Russia has already rejected a Western-backed draft. Moscow has used its veto seven times to block UN resolutions critical of its ally Syria.
- BBC / Reuters / RNZ