PARIS (Reuters) - France warned its citizens in Chad on Thursday to be extra vigilant after its fighter jets struck a heavily-armed rebel convoy that crossed last week from southern Libya aiming to destabilise President Idriss Deby.

French warplanes destroyed about 20 pick-up trucks in a third day of air strikes on Wednesday after Deby asked Paris for support. France sees him as key to a wider regional fight against Islamist militants.

“Given the air operations which the French military are carrying out in northeast Chad and to prevent any malicious act against them, French nationals are urged to be more vigilant,” the embassy said in a message to its 1,500 citizens in Chad.

The strikes, which started on Sunday, come as Chadian rebels have increased their activities in southern Libya since vowing last year to overthrow Deby.

The Union of Forces of Resistance (UFR), a rebel Chadian coalition created in 2009 after almost toppling Deby, has said it was behind this week’s incursion, which saw some 50 pick-up trucks drive almost unopposed more than 600 km (372.82 miles) into Chadian territory.

“Bombings are continuing today. Several dozen vehicles have been destroyed and there are many seriously wounded,” a senior UFR official told Reuters, declining to say how many had died.

“We will not return to Libya. We will wage war inside Chad.”

Army spokesman Patrik Steiger said there had been no French air strikes on Thursday and the military was assessing the state and intentions of the convoy.

A statement after a Chad government meeting said Deby had informed ministers “the column of mercenaries” had been completely destroyed.

Deby has faced several rebellions since seizing power in 1990 in a military coup. International observers have questioned the fairness of elections that have kept him in office since, and last year he pushed through constitutional reforms that could keep him in office until 2033.

France intervened in 2008 to stop the UFR toppling Deby, but President Emmanuel Macron has said he wants a new relationship with France’s former colonies and the era of propping up leaders is over.

However, France considers its former colony as vital in the fight against Islamist militants in West Africa and based its 4,500-strong counter-terrorism Operation Barkhane force in the capital N’djamena

The incursion, which Chadian troops had initially attempted to stop, underscores how Deby’s fight against Islamist militants in the region has strained his military and hit the oil-dependent economy, leading to growing dissatisfaction in one of the world’s poorest nations.