Nato says it has taken over all air operations over Libya from the US, which had led the international force that has been bombarding Muammar Gaddafi's forces.
Nato secretary-general Anders Fogh Rasmussen said the alliance assumed sole command of all air operations over Libya at 6am.
The operation - codenamed Unified Protector - now includes enforcement of the no-fly zone, maintaining the arms embargo on Libya, and the protection of civilians from attacks by Gaddafi's military.
Mr Fogh Rasmussen said that, in line with the UN Security Council mandate, Nato's operational focus would be "on protecting civilians and civilian-populated areas against the threat of attack."