US envoy to NATO: Alliance lacks ‘sound legal basis’ for Syria intervention

Daadler explained that during the uprising in Libya last year, which many have compared to the crisis in Syria, NATO agreed that three criteria must be met before a decision to intervene militarily could be reached. Only one of those three has been met with regards to Syria, said Daadler.

"First there had to be a demonstrable need for intervention. Clearly in the case of Syria there is a real need, given the indiscriminate shelling by government forces of the civilian population," Daadler said in an interview with the Royal United Services Institute Wednesday.

In addition, "clear regional support" and a "sound legal basis" for intervention also had to be found before NATO acted in Libya, neither of which are present in the case of Syria, said Daadler.

The NATO ambassador pointed to the Arab League's call for action in Libya, as well as member country involvement in military operations, as an example of sufficient regional support. While Arab League members have backed a peace plan offered by former United Nations Secretary General Kofi Annan, there has been no unity on military intervention in Syria.

Daadler also said that in order to get a 28-country NATO consensus, the legal justification would "most likely" have to be a UN Security Council resolution. 

"We don't have, in the case of Syria, a call for intervention by the region, indeed there's no call for military intervention by the opposition as such, so we don't have regional support and we lack a sound legal basis, given that the U.N. Security Council has not acted to enforce a mandate for military action," he said. 

Friday's massacre of more than 100 civilians in Syria sparked renewed calls for the use of military force and prompted the United States and others to expel Syrian diplomats in protest.

GOP presidential contender Mitt Romney called on the administration to arm the opposition forces. The White House, however, has warned that further "chaos and carnage" could erupt by sending weapons to anti-Assad rebels.

Daadler called for increased economic and political pressure to push Assad from power. 

"We need to have a political solution to this conflict in order to get that political solution we need to increase the pressure on the Assad regime," he said.