"The UN condemns in the strongest terms the air strike... on Metiga airport" in Tajoura, east of Tripoli, Leon said.
"It is an extremely negative move and, of course, it is unacceptable," he said in Morocco ahead of a new round of peace talks Thursday.
"We hope that an explanation will be given to the international community."
Libya's internationally recognised government said in March that loyalist forces had launched an offensive to "liberate" Tripoli and its suburbs, and its warplanes have bombed militia targets in the past.
Libya has been gripped by chaos and violence since its 2011 revolution that toppled and killed veteran dictator Moamer Kadhafi. It has had rival administrations and parliaments since an Islamist-backed militia alliance seized the capital in August, prompting the internationally recognised government to take refuge in the east.