French authorities believe the 21-year-old was murdered by her ex-boyfriend, who is believed to have fled to Chile
Narumi Kurosaki went missing in the city of Besancon, where she had been studying French, on the night of December 4.
French authorities believe the 21-year-old was murdered by her ex-boyfriend, who is believed to have fled to Chile, and have issued an international warrant for his arrest.
Chile's justice ministry said Wednesday it was assisting the French "in the identification of certain suspects as of their latest movements", without giving further details.
In a video published by French and Japanese media, the suspect, identified as Nicolas Zepeda Contreras, threatens Kurosaki with unnamed consequences if she did not fulfil unspecified conditions.
Speaking in English in a video dating from September, he says he loves Kurosaki, whom he identifies by first name, but accuses her of doing "bad things".
Holding up two fingers he gave her a two-week deadline until September 21 to meet his conditions, which he said applied "some for her whole stay in France and some, actually, forever."
"She needs to build trust and she needs to pay a little cost of what she did and assume (sic) that she cannot go around making those kinds of mistakes with a person who loves her," he added.
The case has caused shock in Japan as well as in France.
Kurosaki's fellow students in Besancon described Kurosaki as a "lovely person" who was "always smiling".
On Thursday, police with sniffer dogs searched a vast forest on the outskirts of Besancon for the body of the petite student.
Le Parisien newspaper reported that signals from the suspect's phone had been traced to Chaux forest.
The prosecutor's office and police refused to confirm the report.
On Tuesday, Besancon prosecutor Edwige Roux-Morizot said the investigation had shown the student's Chilean ex "was in France, in Besancon, at the time of her disappearance".
"The evidence tells us it is extremely likely that the young woman is dead," she said.
The prosecutor ruled out suicide, describing Kurosaki as a "young girl full of life" who was happy in a new relationship.
"There is sufficient evidence to launch a murder investigation," she said, describing the suspect as having an "intrusive, disturbing personality".
Kurosaki, who had been studying in Besancon since September, met the Chilean teaching assistant in Japan but the pair later split.
Several students living on the same floor of her university residence said they heard a loud cry on the night of December 4.
Anne-Laure Saillet, 20, told AFP she heard "bangs on the doors and furniture and afterwards I heard a very shrill cry, very sharp and loud. It was a woman's cry.
"At first I thought it was a horror film but when it continued I became worried."
Roux-Morizot said Tuesday that tests were being carried out on reddish traces found on the steps of the building to determine whether they were bloodstains.