Beirut: The Syrian army said a US missile attack on one of its airbases on Friday killed at least nine civilians, including four children, and caused extensive damage, adding it would respond by continuing its campaign to "crush terrorism" and restore peace and security to all of Syria.

A statement from the army command described the attack as an act of "blatant aggression", saying it had made the United States "a partner" of Islamic State, the ex-Nusra Front and other "terrorist organisations."

The office of Syria's President Bashar Assad, meanwhile, has called the US missile strike against the air base in central Homs "reckless" and "irresponsible," Associated Press is reporting.

The strikes, it said, were "shortsighted" and not based on true facts. The US strike followed Tuesday's gruesome chemical attack in the town of Khan Sheikhoun, where more than 80 people were killed.

The US, under order of President Donald Trump, fired dozens of cruise missiles at an air base from which it said the deadly chemical weapons attack was launched this week. That attack on the town of Khan Sheikhoun killed more than 80 people, including some 30 children.

This is the first time the US has directly targeted Syrian President Bashar Assad's forces. The Obama administration threatened to attack Assad's forces after previous chemical attacks, but did not. Trump called on "all civilised nations" to join the US in seeking an end to the carnage in Syria and said he had ordered the strikes in America's "national security interest".

From his his Mar-a-Lago estate in Florida, Trump said his decision had been prompted in part by what he called failures by the world community to respond effectively to the Syrian civil war.

"Years of previous attempts at changing Assad's behaviour have all failed, and failed very dramatically. As a result, the refugee crisis continues to deepen, and the region continues to destabilise, threatening the United States and its allies."

United Nations Secretary General Antonio Guterres appealed to parties involved in the Syrian conflict for restraint to avoid adding to the suffering.

"Mindful of the risk of escalation, I appeal for restraint to avoid any acts that could deepen the suffering of the Syrian people," he said.

According to the The situation in Syria now "amounts to an international armed conflict" following the US missile strikes, according to the International Committee of the Red Cross.

"Any military operation by a state on the territory of another without the consent of the other amounts to an international armed conflict," ICRC spokeswoman Iolanda Jaquemet said in Geneva.

"So according to available information – the US attack on Syrian military infrastructure – the situation amounts to an international armed conflict."

Russian state TV said the air strikes had destroyed nine planes, but had left the main runway relatively unscathed.

The Rossiya 24 channel showed footage of the base after it was struck by US cruise missiles. Craters, debris and rubble was visible, but the main runway looked largely intact. It was unclear whether the channel was showing all or just part of the base.

The Syrian army said the attack "undermines the operation of combating terrorism that the Syrian Arab Army is carrying out".

"The United States has tried to justify this aggression under the pretext of the Syrian Arab Army using chemical weapons in Khan Sheikhoun, without knowing the reality of what happened," it added.

In Moscow, Russia said it would withdraw from a pact with Washington to share information about warplane missions over Syria, where a US-led coalition is also waging airstrikes on Islamic State targets.

Russian President Vladi­mir Putin called for an immediate meeting of the UN Security Council and his spokesman, Dmitry Peskov, called the US missile strikes "violations of the norms of international law, and under a far-fetched pretext."

The Kremlin claim only 23 of 59 cruise missiles reached the air base, destroying six Syrian jets but leaving the runway intact. Moscow also confirmed it had been informed of the attack in advance. A US-led coalition has been bombing Islamic State targets in Syria since 2014, while Russia's air force has been striking both extremist groups and Syrian rebels in order to aid Assad's forces.

In the US, politicians from both parties backed Mr Trump's missile strikes, while urging him to spell out a broader strategy for dealing with the conflict.

"I am hopeful these strikes will convince the Assad regime that such actions should never be repeated," said Senator Mark Warner, a Virginia Democrat.

But Warner, who said he had been briefed on the strikes by Director of National Intelligence Dan Coats, urged Trump, a Republican, to lay out his plans for the multi-sided Syria conflict.

"President Trump has said repeatedly that his objective in Syria is to defeat [Islamic State militants]. Last night's strike was aimed at a different objective," he said.

"President Trump needs to articulate a coherent strategy for dealing with this complex conflict, because the consequences of a misstep are grave."

Armed Services Committee chairman, Senator John McCain, who has long called for more aggressive action against Assad, said "the signal I think that was sent last night ... was a very, very important one."

Trump, he said, should be "prepared to take other action," including establishing safe zones within Syria and further arming and training of anti-Assad rebels.

Several legislators said Trump should seek Congress' approval if he decides to take additional military action in Syria.

US allies around the world expressed support, if sometimes cautiously, of Washington's strikes on Syria.

Prime Minister Malcolm Turnbull said the strikes sent "a vitally important message" that the world would not tolerate the use of chemical weapons.

"The retribution has been proportionate and it has been swift," he said. "We support the United States in that swift action."

Britain, France and Japan all expressed support.

"The UK government fully supports the US action, which we believe was an appropriate response to the barbaric chemical weapons attack launched by the Syrian regime and is intended to deter further attacks," a British government spokesman said.

Japan's Prime Minister Shinzo Abe told reporters: "Many innocent people became victims from the chemical attacks. The international community was shocked by the tragedy that left many young children among the victims. Japan supports the US government's determination to prevent the spread and use of chemical weapons."

French Foreign Minister Jean-Marc Ayrault said Russia and Iran needed to understand that supporting Assad made no sense and that the escalation of the US military role in Syria was a "warning" to "a criminal regime".

Turkey's Foreign Minister called for Syrian President Bashar al-Assad's immediate removal, saying a transitional government must be established and voicing support for the air strike.

"It is necessary to oust this regime as soon as possible from the leadership of Syria," Foreign Minister Mevlut Cavusoglu told reporters in the southern town of Alanya in comments broadcast live.

"If he doesn't want to go, if there is no transition government, and if he continues committing humanitarian crimes, the necessary steps to oust him should be taken," he said.

Cavusoglu said safe zones for civilians in Syria were now more important than ever. He said the coalition had been informed of the US missile strike and that he had spoken by phone with the French and German foreign ministers, although he did not say when.

He also said contacts had been initiated with Russian Foreign Minister Sergei Lavrov and US Secretary of State Rex Tillerson.

Washington has blamed Assad's forces for the chemical attack. The Syrian government has strongly denied responsibility and blamed the deaths on leaks from a rebel chemical arms store it says was hit by a Syrian air strike.

Rebels say there were no military positions in the targeted area in Khan Sheikhoun and that they have no chemical weapons.

The army said Friday's US attack "sends the wrong messages to the terrorist organisations and makes them go further in using chemical weapons in the future whenever they face big losses on the battlefield".

The Syrian government describes all armed groups opposed to it as terrorists.

Reuters, Associated Press