MONTREAL (AFP) - Some 40 schools were closed on Tuesday in Quebec over bomb threats, the Canadian province's public safety minister said.

An exhaustive police search turned up no explosives and many of the schools were reopened by midday.

Others suspended classes.

Quebec Public Safety Minister Pierre Moreau said a "group" unknown to police had made the threats in an e-mail sent to school administrators in Quebec and neighbouring Ontario province, saying they were opposed to the curriculum.

Quebec police were informed about it overnight, he said.

Both Quebec and Ontario have been gripped by labour strife as they seek to negotiate new collective agreements with teachers.

The bomb threat was likely a hoax, according to Moreau, "but it was taken seriously" by Quebec police who are now jointly investigating with the Royal Canadian Mounted Police.