The United Nations Security Council has in the past declared no-fly zones over conflict regions. https://www.un.org/press/en/2011/sc10200.doc.htm
But, what troops does the UN use to enforce this given that the peacekeeping forces it has are very limited in number? Or is this just a call to action on member states to use their military to enforce the UNSC's declaration?
The Security Council authorises the use of force by nations (typically led by the permanent members of the S.C, and particularly the USA). The UN doesn't have troops of its own.
In the particular example, Resolution 1973 of the council:
Authorizes Member States that have notified the Secretary-General and the Secretary-General of the League of Arab States, acting nationally or through regional organizations or arrangements, to take all necessary measures to enforce compliance with the ban on flights imposed by paragraph 6 above, as necessary, and requests the States concerned in cooperation with the League of Arab States to coordinate closely with the Secretary General on the measures they are taking to implement this ban,
In other words, normally, implementing a no-fly zone over a sovereign nation would be an act of war (and so illegal under international law, unless in self-defence). But this resolution explicitly makes implementing a no-fly zone legal. It makes it legal (in international law) for American aircraft (for example) to shoot down any Libyan plane in the sky over Libya.
It's a bit misleading to think the United Nations (UN) only has “limited” peacekeeping ressources. The reality is that it does not have any military force that could be freely dispatched anywhere in the world under UN command. None at all. That may have been the original intent but it never happened. Instead, the way UN peacekeeping works is that countries provide troops for specific missions. Nations providing troops often get money for this, paid from other nations' financial contributions.
On the other hand, if you include all soldiers currently deployed for the UN, the total size of peacekeeping operations is actually quite large (currently over 100 000 personnel) and growing. And it's only limited by UN member states willingness to provide military assets and funding for each mission so that UN security council (UNSC) members could probably find ressources if they really wanted to create a new one.
Another model is for some countries to deploy forces directly to enforce some UN resolution but without formally belonging to a UN peacekeeping operation (that also means no blue helmet and a different chain of command). In particular, the US does not provide significant forces to the UN presumably because they do not want US troops under foreign command (they do provide a lot of funding).
When the US does support peacekeeping operations on the ground, they typically do it alongside UN troops (e.g. in Somalia) rather than as a part of the UN operation. Enforcing a no-fly zone requires very specific capabilities and I think it has always been done directly by a handful of countries (US, UK, and other NATO members) following the same model, even when it's done under a UNSC mandate.
