UN Secretary General Ban Ki-Moon said, on Tuesday, that the use of force was illegal unless it was for self-defence or authorised by the UN Security Council.
Analysts said that Moon's remarks looked as though he was sceptical about the legality of America's plans to attack Syria without the support of the UN.
Moon said that if the UN inspectors confirmed the use of chemical weapons, the UNSC had to overcome its differences and take measures to respond.
"If the use of chemical weapons by one side against the other was confirmed, that will be a dangerous violation of international law and a hideous war crime," he told journalists. "Perpetrators of those atrocities must be brought to justice and must not escape punishment."