Bana Alabed disappeared from the social network on Sunday as a government offensive was launched on rebel-held areas in eastern Aleppo.
However, a new tweet from her mother on Monday suggested there was still grave danger and that there was "nowhere to go".
The account, which details life in the war, has more than 160,000 followers.
Many have expressed concern about her safety.
A BBC source says both she and her mother, Fatemah, who often tweets on her behalf, are in an undisclosed location.
However, a new tweet from Fatemah on Monday read: "Under attack. Nowhere to go, every minute feels like death. Pray for us. Goodbye - Fatemah #Aleppo".
Author JK Rowling was among those who used Twitter to call for information about Bana's whereabouts on Sunday.
Ms Rowling sent Bana an entire set of Harry Potter eBooks after the young girl posted that she liked to read "to forget the war".
In a conversation with the BBC in October, Fatemah said her daughter wanted "the world to hear our voice".
The family, which includes Bana's two younger brothers, were believed to live in rebel-held eastern Aleppo, which has come under heavy bombardment from the army in recent weeks.
A seven-year-old girl whose tweets from a besieged Syrian city drew worldwide attention is safe, the BBC has learned.