Government officials rushed journalists to a destroyed building and later to a hospital, where they showed them four people the government says were killed in the strike.
The allegations have not been independently verified. NATO has repeatedly insisted it tries to avoid killing civilians.
A NATO spokesman said it was investigating and added that the Libyan government has made false claims about NATO causing civilian deaths in the past.
Saturday, NATO announced that it had mistakenly struck a column of Libyan rebel vehicles in an airstrike near an eastern oil town two days earlier and expressed regret for any casualties that might have resulted.
The alliance has accidentally hit rebel forces before in its air campaign to protect civilians in the civil war between Moammar Gadhafi's military and the fighters trying to end his more than four decades in power.
A coalition including France, Britain and the United States launched the first strikes against Gadhafi's forces under a United Nations resolution to protect civilians on March 19. NATO, which is joined by a number of Arab allies, assumed control of the air campaign over Libya on March 31.
The Associated Press contributed to this story