SEVARE, Mali — Islamist extremists based in the Malian town of Ansongo destroyed a bridge near the Niger border, officials said on Friday, marking the first use of explosives by the insurgents since the start of a French-led military intervention two weeks ago.

The explosion shows that the extremists remain a nimble and daunting enemy, despite gains by the French, who have recaptured three towns and on Friday pushed toward the Islamist stronghold of Gao, one of three provincial capitals controlled by the Qaeda-linked rebels.