A jihadist-linked news agency released a video on Thursday showing one of the France church attackers threatening the country, a monitoring group said.
Abdel Malik Petitjean, 19, was identified by French authorities on Thursday as the second jihadist in the brutal killing of an elderly priest in Normandy on Tuesday.
He was added to a French security watch list last month after trying to reach Syria from Turkey.
The video released by the Amaq news agency showed Petitjean, dressed in a green striped t-shirt, speaking into the camera from what appeared to be a home.
The SITE Intelligence Group, which monitors extremist communications online, provided a copy of the two-and-a-half minute video.
In it, Petitjean makes threats against France and directly addresses French President Francois Hollande and Prime Minister Manuel Valls.
He speaks mostly in French but uses some Arabic phrases.
France has been a prime target of Islamic State, which regularly calls for supporters to launch attacks there.
Petitjean and Adel Kermiche, also 19, pledged allegiance to the Islamic State group in a video made before they stormed a church in the Normandy town of Saint-Etienne-du-Rouvray on Tuesday and slit the throat of 86-year-old priest Jacques Hamel at the altar.
Both assailants were shot dead by police.
The attack came as the government was already facing a firestorm of criticism over alleged security failings after the Bastille Day truck massacre in Nice that left 84 people dead two weeks ago.
Calls have been mounting for Valls and interior minister Bernard Cazeneuve to resign.