World leaders have been responding to US President Donald Trump's overnight missile strikes on a Syrian government air base suspected of launching a chemical attack on a rebel-held town on Tuesday.
President Bashar al-Assad's office has spoken out against the US airstrikes.
"What America did is nothing but foolish and irresponsible behaviour, which only reveals its short-sightedness and political and military blindness to reality," it said.
The Syrian National Coalition - Assad's main opposition - said it wanted to see more air strikes to impair the Assad government's abilities.
"We hope for more strikes... and that these are just the beginning," spokesman Ahmad Ramadan told the AFP news agency.
But the Free Syrian Army told Reuters it feared for acts of "revenge" by Assad, and his allies, against civilians.
Dmitry Peskov, spokesman for the Russian President Vladimir Putin, described the US air strikes on the Shayrat airbase as "an act of aggression against a sovereign state delivered in violation of international law under a far-fetched pretext".
His statement said Washington's actions had "dealt a serious blow to Russian-US relations, which are already in a poor state".
The strikes would "create a major obstacle to the establishment of an international counterterrorist coalition", it added.
Russian Foreign Minister Sergei Lavrov called it "an act of aggression under a completely invented pretext".
"Everything resembles the situation of 2003, when the USA, the UK and several of their allies invaded Iraq without the UN Security Council's approval - a grave violation of international law - but at that point they at least tried to show some material evidence."
The UK government said it fully supported the strikes and had been in close contact with the US government beforehand.
Defence Secretary Michael Fallon told the BBC: "The Americans believe they've exhausted all possible diplomatic and peaceful ways of dealing with the use by the regime of chemical weapons and they have been determined to try to prevent future attacks like this so they've taken this action today."
He said the US "hasn't declared war" on Syria and the UK had not been asked to get involved.
Jeremy Corbyn, who leads the opposition Labour Party, condemned the attack, saying that it "risks escalating the war in Syria still further".
The President of the European Council, Donald Tusk, tweeted his reaction to the unilateral airstrikes.
Jean-Claude Juncker, President of the European Commission, said in a statement: "The US has informed the EU that these strikes were limited and seek to deter further chemical weapons atrocities."
"The repeated use of such weapons must be answered."
But the union's foreign policy chief issued a more critical response on behalf of all the member states.
Reuters quoted Federica Mogherini saying that while the US had an "understandable intention to prevent and deter the spread and use of deadly chemical weapons", those responsible should face justice "within the framework of the United Nations".
Jens Stoltenberg, Secretary General of defence alliance Nato, said in a statement: "The Syrian regime bears the full responsibility for this development.
"Any use of chemical weapons is unacceptable, cannot go unanswered, and those responsible must be held accountable."
France and Germany
German Chancellor Angela Merkel and French President Francois Hollande issued a joint statement in response to the US air strike.
It said: "President Assad alone bears the responsibility for this development. His repeated use of chemical weapons and his crimes against his own people demand sanctions which France and Germany already asked for in the summer of 2013 after the massacre at Ghouta."
Angela Merkel later added that the attack was "understandable in view of the extent of the war crime" but "it remains right and important to focus all strength on political talks".
A spokesman for the Iranian Foreign Ministry, quoted by ISNA news agency, said: "Iran strongly condemns any such unilateral strikes ... such measures will strengthen terrorists in Syria... and will complicate the situation in Syria and the region."
Israel's Prime Minister Benjamin Netanyahu said in a statement: "President Trump sent a strong and clear message today that the use and spread of chemical weapons will not be tolerated."
"Israel fully supports President Trump's decision and hopes that this message of resolve in the face of the Assad regime's horrific actions will resonate not only in Damascus, but in Tehran, Pyongyang and elsewhere."
Saudi Arabia has said it fully supports the US military strikes, praising what it described as "the courageous decision" by US President Trump.
Turkish President Recep Tayyip Erdogan, who is an opponent of President Assad's, welcomed the air strike.
"I want to say that I welcome this concrete step as positive," he said. "Is it enough? I don't see this as enough... the time has come for steps for a serious result to protect the oppressed Syrian people," he added.
His spokesman also called for the creation of a no-fly zone and safe zones within Syria.
Turkey's foreign ministry added that Turkey would fully support steps that would ensure accountability for the Syrian regime.
"What is urgent now is to avoid further deterioration of the situation," foreign ministry spokeswoman Hua Chunying said during a regular press briefing.
"We oppose use of chemical weapons by any country, organisation or individual in any circumstance, for any purpose."
The Japanese government has said it supports the US government's determination to oppose the spread and use of chemical weapons.
Prime Minister Shinzo Abe said: "We understand the US government's strikes this time are to prevent further deterioration of the situation."
He said he valued the president's "strong commitment" to "maintaining international order as well as peace and security with US alliances and the world".
"The Australian government strongly supports the swift and just response of the United States," Prime Minister Malcolm Turnbull said.
"This was a calibrated, proportionate and targeted response. It sends a strong message to the Assad regime."
Italian Prime Minister Paolo Gentiloni said that the US strikes were a "motivated response to a war crime" that the Syrian regime was responsible for.
Mr Gentiloni added that he hoped the strike "should accelerate chances of political negotiations for a long lasting solution" to the Syrian crisis.
Poland's President Andrzej Duda expressed his full support for the military operation in a statement.
"President Donald Trump's decision to attack one of the Syrian air bases was a reaction to the use by Assad's military regime of chemical weapons against civilians. The civilised world could not be indifferent to this act of unimaginable barbarity."
"Canada fully supports the United States' limited and focused action to degrade the Assad regime's ability to launch chemical weapons attacks against innocent civilians, including many children," Prime Minister Justin Trudeau said in a statement.
"President Assad's use of chemical weapons and the crimes the Syrian regime has committed against its own people cannot be ignored."