Technically, yes, thanks to a vague U.N. resolution.
The governments enforcing the no-fly zone over Libya are currently deadlocked over who will coordinate the international effort. The United States and Britain are pushing for NATO to take over, while France is advocating a "political steering body" to manage the mission, in order to make sure that Arab governments remain involved (at least superficially). Part of the problem comes from the vagueness of U.N. Security Council Resolution 1973, which establishes the no-fly zone, but avoids specifying which countries will lead or participate in its enforcement. So far, the mission has been led by the United States, Britain, and France -- with Canada, Belgium, Denmark, Italy, Norway, Spain, and Qatar also participating. But, in theory, could any country that wants to take it upon itself to enforce the no-fly zone?
Technically, yes. Security Council Resolution 1973 "authorizes Member States that have notified the Secretary-General, acting nationally or through regional organization and arrangements, and acting in cooperation with the Secretary-General to take all necessary measures ... to protect civilians and civilian populated areas under threat of attack in the Libyan Arab Jamahiriya." It goes on to prohibit a "foreign occupation force" on Libyan territory and "requests that the Member States ... inform the Secretary-General immediately of the measures they take pursuant to the authorization."
So if, say, Palau decided to start flying reconnaissance sorties over Benghazi this week there's nothing in the resolution to stop it. (This would admittedly be difficult, since Palau doesn't have a military.) The drafters of the resolution may have intentionally left this passage vague in order to avoid giving any one regional organization, such as NATO or the Arab League, responsibility for enforcing the resolution.
Moreover, the phrase "acting in cooperation with the Secretary-General" is vague enough so that member states wishing to participate in the military action don't actually have to get approval from Ban Ki-moon's office, they just have to inform him of their participation.
The vagueness of the resolution isn't particularly unusual for actions taken under the U.N. Charter's Chapter 7, which authorizes member states to use force in response to "threats to the peace, breaches of the peace, and acts of aggression." Resolution 678, which began the 1991 Gulf War, was actually even less specific, authorizing "Member States co-operating with the Government of Kuwait" to "use all necessary means to uphold and implement resolution 660." The Libya resolution at least asks that the secretary general be informed and places limits on the types of military action that can be taken and where.
As one might expect, member states often disagree on just what a Security Council resolution actually authorizes them to do. The United States and several allies read 678 as granting them authority to enforce a no-fly zone in Iraq after major combat operations had ended in order to protect civilians, an interpretation that was widely disputed at the time.
And in the Libya campaign, rifts are already beginning to show. U.S. and British officials were reportedly angered that France launched the first airstrikes against Libya without consulting them. And aside from two Qatari figher planes and a cargo jet that are currently en route, Arab countries are not participating to a significant degree, despite a paragraph in the resolution that specifically "recognizes the importance of the League of Arab States in matters relating to the maintenance of international peace and security in the region."
Not than anything's stopping more countries from joining in, should they want to.
Thanks to Micah Zenko, fellow for conflict prevention at the Council on Foreign Relations; Anthony Clark Arend, professor of government and foreign service at Georgetown University; Philip G. Alston, professor of law at the New York University School of Law; and Jose Alvarez, professor of international law at the New York University School of Law.