French warplanes destroyed about 20 pick-up trucks in a third day of air strikes on Wednesday against a Chadian rebel convoy that crossed last week from Libya, an operation the French military said was aimed at preventing the destabilization of its former colony.

The strikes, which started on Sunday, come as Chadian rebels have increased their activities in southern Libya since vowing last year to overthrow President Idriss Deby. The strikes were also held on Tuesday but not Monday.

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 400 km (250 miles) into Chadian territory.

“The incursion of this armed column deep into Chadian territory was aimed at destabilizing this country,” the French military said in a statement.

The strikes by Mirage fighter jets were carried out in response to a formal request for assistance by a sovereign state and were conducted according to international humanitarian law, it said. The planes took off from the Chadian capital N’Djamena and were supported by a Reaper drone, the statement said. A UFR official told Reuters on Monday two of its fighters had been killed.

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 Chad as crucial given it is deemed as having the most battled-hardened troops in the fight against Islamist militants in West Africa. Paris has based its 4,500-strong counter-terrorism Operation Barkhane force in N’Djamena, where the United States also has a base.

The “Chadian army is an essential partner in the fight against terrorism in Mali … the G5 force and its action against Boko Haram,” the French military statement said.

Chadian air strikes had initially attempted to destroy the rebel convoy on February 1-2. 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.