The general assembly of the United Nations could pass a resolution condemning the attack. In the past, it has passed for example Resolution 47/121 condemning ethnic cleansing of the Bosnian Muslims by the Bosnian Serbs. Such resolutions can be passed by a simple majority, or a two thirds majority if the issue is considered important. General assembly resolution by themselves do however rarely translate in to impactful actions, such as military interventions.
More effectual resolutions can be passed by the United Nations Security Council. Such resolutions have in the past lead directly to military interventions, for example Resolution 743, leading to the establishment of a peacekeeping mission in Yugoslavia under a UN banner, and Resolution 1973, leading to military intervention in Libya.
However, United Nations Security Council resolutions can be vetoed by any of the five permanent members of the Security Councils. In the case of Syria, the position of Russia as a permanent member would likely prevent any resolution that could be a grounds for anti-regime action, as Russia has vested interests in the continuation of the Assad regime.