Roadblocks have been set up across north-west France, with police following up on every tip-off.

Speaking to the media following a crisis unit meeting, Mr Dupond-Moretti confirmed that two officers had died.

"One leaves behind a wife and two children who were meant to celebrate their 21st birthday in two days. The other leaves a wife who is five months pregnant," he said.

"Everything - and I mean everything - will be put in place to find the perpetrators of this vile crime," Mr Dupond-Moretti said, adding that the perpetrators were "people for whom life has no value".

They would be found and punished "in a way that is proportionate to the crime", he said.