John B. Bellinger III is a partner with Arnold & Porter LLP and an adjunct senior fellow in international and national security law at the Council on Foreign Relations. He served as legal adviser to the State Department from 2005 to 2009.

The escalating death toll in Syria, which exceeds 60,000, has increased pressure on President Obama to do more to help the Syrian opposition. But traditional legal rules that protect international peace and security constrain the president’s options. Although the administration recognized the Syrian Opposition Council last month as the “legitimate representative of the Syrian people,” that announcement created no new legal basis for Washington to give weapons to Syrian rebels or to intervene with military force against the Assad government. If Bashar al-Assad’s atrocities continue, Obama will find it difficult to provide more U.S. assistance consistent with international law.

The U.N. Charter prohibits member states from using force against or intervening in the internal affairs of other states unless authorized by the U.N. Security Council or justified by self-defense. These rules make it unlawful for any country to use direct military force against the Assad regime, including establishing “no-fly zones” or providing arms to the Syrian opposition without Security Council approval. Russia and China, of course, have continued to block such approval.

Over the years, people in and outside government advocating U.S. intervention — whether in Syria, Libya or other circumstances — have chafed at these restrictions. Many human rights advocates believe that traditional non-intervention principles unduly constrain efforts to prevent mass atrocities; some have urged recognition of a new international norm that would allow “humanitarian intervention” in such cases. But most countries, including the United States, have resisted a doctrine of humanitarian intervention because it is likely to be abused by potential aggressors.

Many conservatives have long been skeptical of international legal rules that purport to restrict sovereign nation-states’ freedom of action. In an unusual convergence, these conservatives agree with liberal human rights advocates that international law should not prevent the use of force or other forms of intervention to help the Syrian opposition.

In the president’s own words, his endorsement of the Syrian Opposition Council on Dec. 11 was a “big step.” His announcement followed similar endorsements by Britain, France, Turkey, the European Union and the seven-country Gulf Cooperation Council and paved the way for increased U.S. humanitarian aid to the opposition. Such aid reached more than $210 million last month.

But Obama stopped short of recognizing the opposition as the “government” of Syria. The United States continues to maintain diplomatic relations with Damascus and to treat the Assad regime as Syria’s government.

In July 2011, by contrast, the administration recognized the Libyan opposition as the “legitimate governing authority in Libya.” That represented an unusual departure from past U.S. diplomatic practice of recognizing “states” but not “governments,” especially in the face of competing claims. Recognizing the Libyan opposition as that country’s government allowed the Obama administration to unfreeze substantial Libyan-government assets in the United States and to turn them over to the opposition.

Treating the Syrian group as the “representative” of the people does not have a similar legal effect and does not allow the United States to treat the opposition’s requests for aid or support as a legal basis for U.S. military intervention.

Obama’s caution about intervening directly in Syria or arming the opposition has been prudent. Intervention without an international legal basis could make it more difficult for Washington to criticize other countries if they intervene in neighboring states based on less laudable motives. Inserting more arms into an already unstable region risks more bloodshed, and those weapons could fall into the hands of groups hostile to U.S. interests, as happened in Libya.

As the violence in Syria increases, however, the president is likely to feel compelled to provide more than political support and non-lethal aid. If the Syrian Opposition Council becomes more inclusive and can legitimately claim to represent the majority of Syrians, and if it excludes terrorist groups and other extremists, the administration may conclude that it is legally permissible to provide military assistance based on the council’s consent. But other countries may disagree with this rationale. Alternatively, the administration could intervene in a limited way to protect civilians without asserting a legal basis, as the Clinton administration did with its participation in the 1999 NATO bombing campaign of Kosovo, to protect Kosovars from atrocities committed by Serbia. The Kosovo intervention was viewed by most international lawyers as legitimate, although not strictly legal.

Humanitarian crises challenge international legal rules as well as our consciences. But when the Security Council is blocked from protecting civilians against the most egregious atrocities, the United States should be prepared to intervene when other avenues have been exhausted and there is sufficient international consensus to support intervention. If Assad’s attacks on Syrian civilians continue, the United States and other governments may soon conclude that intervention is morally, if not legally, justified.