"The use of military force against a state, as a sanction against the violation of an international convention by this state, is an infringement of the prohibition of the use of violence in international law," said the Bundestag experts in a reply to a question filed by the far-left Die Linke party.

The Bundestag experts referred in particular to the United Nations declaration from their 1970 general assembly which stresses "the duty of States to refrain in their international relations from military, political, economic or any other form of coercion aimed against the political independence or territorial integrity of any State".

The UN Security Council had also rejected armed retaliation, calling it "incompatible with the objectives and the principles of the United Nations."

The legal motive put forward by Britain, which joined in the air strikes alongside the United States and France, was also "not convincing," said the Bundestag experts.

London said it was "both right and legal" to launch strikes to alleviate humanitarian suffering.

But the experts said there were questions over "whether the military attacks are really appropriate to prevent further suffering" in Syria.

On April 14, the United States, France and Britain fired missiles meant as a response to what the trio of nations said was evidence Damascus had used chemical weapons a week earlier.

Purported footage of victims foaming at the mouth after the April 7 attack sparked an outcry and prompted the West to launch its biggest military action yet against Assad's regime.

But the military action was carried out without the approval of the UN Security Council, where Assad's ally Russia has a permanent seat and veto.

Germany did not join in the air strikes but Chancellor Angela Merkel called the military action "necessary and appropriate".