France reintroduced border checks on Friday as it tightened security ahead of UN climate talks that start in Paris at the end of the month.

France reintroduced border checks on Friday as it tightened security ahead of UN climate talks that start in Paris at the end of the month.

Controls will be in place until December 13 at 285 road, rail, sea and air border points.

About 30,000 police will be involved in the operation.

US President Barack Obama, his Chinese counterpart Xi Jinping and Indian Prime Minister Narendra Modi are among more than 115 heads of state and government attending the 21st United Nations Climate Change Conference to be held at Le Bourget outside Paris from November 30 to December 11.

The conference aims to agree a new global pact to fight climate warming.

French Interior Minister Bernard Cazeneuve has said the risk of "terrorist threat (or) public disturbance" during the conference cannot be dismissed.

French security sources are paying particular attention to militant ecologist protesters already active around the site of a new airport under construction at Notre-Dame-des-Landes in western France and potential attempts to disrupt the train line linking the southeast French city of Lyon with Turin in northern Italy, sources said.

The potential involvement of extreme-left militant groups in demonstrations timed to coincide with the climate conference is also being watched closely.

The tightening of France's borders comes as Europe's cherished Schengen zone is buckling under the pressure of the biggest migrant crisis since World War II.

France has said it is able to temporarily abandon its commitments to border-free travel guaranteed under the Schengen agreement in special circumstances such as the climate talks.