The two killings took place within less than 24 hours in the Essonne départment south of Paris.
In the first incident on Monday, a 14-year-old girl died of stab wounds to the stomach during a fight between a dozen youths outside a school in the town of Saint-Cheron, 50km south of Paris.
Six minors aged between 13 and 16 have been arrested over her death.
One of the suspects, who was known to the police, has admitted to delivering the fatal blow with a pocket knife, according to the public prosecutor for the area, Caroline Nisand.
The second stabbing took place on Tuesday in the town of Boussy-Saint-Antoine, 45km away in the east, where a 14-year-old boy died, also “very likely after being stabbed in the stomach”, local authorities told AFP.
A 13-year-old boy was also seriously injured after being stabbed in the throat during a clash between around 30 youths from gangs in two towns on either side of Boussy-Saint-Antoine.
The suspected perpetrator of the stabbing turned himself in to police.
The deaths come after an outcry last month over a video of a 15-year-old in southern Paris suffering a vicious gang beating that left him in a coma.
The state’s representative in the Essonne department, Eric Jalon, told France Info radio on Wednesday that brawls between youth gangs in the area “had increased in number, intensity and gravity”.
Interior Minister Gérald Darmanin, who visited the Essonne on Tuesday, said the greater Paris area was particularly affected by fights between teenage gangs.
He blamed the “mimicry of social networks” where youngsters post videos of rivals being beaten up in order to humiliate their opponents.
Nationwide, however, he noted that the level of youth gang violence had subsided since 2016, when nine youngsters were killed in fights.
Police sources said the number of gangs in the Paris area was also largely unchanged in the past five years, with 46 gangs described as active in Paris and the suburbs.