US President Donald Trump's retaliatory strike against a Syrian airbase violates international law and the American constitution, top lawyers have said.

The cruise missile strike was launched from two US warships on Friday (7 April) in response to a chemical weapon attack in the north-western town of Khan Sheikhoun.

Trump said Syrian President Bashar al-Assad was directly responsible for the use of chemical weapons. However, the Syrian government has firmly denied any part in the attack.

Independent assessments have yet to be carried out to confirm whether the attack was carried out by Assad, Isis or anti-government militants, but Trump authorised the strike on Sharyat airfield within 24 hours of the gas attack.killing nine people including four children.

The missile barrage, which lasted about 35 minutes, killed nine people including four children.

American lawyers have raised a number of the legal concerns about the strike in the legal forum Just Security.

With regards to international law, Brian Egan, a former US Department of State legal advisor, said: "It is essential that the United States articulate its international law justification for the strikes announced by the president [on] Thursday night.

"As reported, the strikes do not appear to fit clearly within any of the widely recognized post-UN Charter international legal rationale for using force (self-defence, UN Security Council authorisation, or host government consent)."

Stephen Pomper, a strategist at the Department of Defense, said: "Thus far, we have seen nothing by way of an international legal justification.

"There was of course no UN Security Council mandate for the action and, as others have noted, the facts that have emerged so far do not seem to support a traditional self-defence justification that would permit force to be used consistent with Article 2(4) of the UN Charter."

However, it was argued such an attack on Syria could be justified based on the precedent set during the 1999 intervention in Kosovo.

Pomper added: "The administration may be tempted to follow the script that the US government adopted after the Kosovo intervention in 1999, where the US intervention also lacked both Council authorisation and a clear self-defence rationale.

"In that case, the State Department famously chose to deem the strikes 'legitimate' without defending them as 'legal'."

Countering that view, however, Fionnuala Ni Aolain, chair in law of the University of Minnesota, among others, said: "While there is precedent for the use of force without Security Council authorization, including by the United States in Kosovo, these actions, Kosovo included, weaken the Security Council and the post-WW2 global system of collective security.

"Despite the flaws of that legal order, a slide into self-justificatory unilateralism by the United States should not be celebrated nor validated."

Shayrat airfield Syria US missiles
A satellite image of Shayrat airfield in Homs, taken on 18 February 2017 DigitalGlobe/Courtesy US Department of Defence/Reuters

With regards to constitutional issues, Louis Fisher, a scholar in residence at the Constitution Project, said: "President Trump has no constitutional authority to unilaterally commit the nation to war against Syria, which is the effect of launching cruise missiles against Syria.

"From 1789 to 1950, presidents came to Congress either for a declaration of war or statutory authority whenever they decided it was necessary to take the country from a state of peace to a state of war.

"In doing so, they complied with the Framers' clear intent that the decision to use offensive force against another country must reside solely in Congress."

However, Fisher added, Trump would not be the first president to violate the Constitution by going to war.

He said: "From Truman's war against North Korea in 1950 to Obama's use of military force against Libya in 2011, presidents have ignored that fundamental constitutional principle.

"Instead of seeking authority from Congress, presidents turn instead to the UN Security Council or Nato allies for support. The result: a series of unconstitutional wars."