A spokesman for the "Free Qalamoun" group told Asharq al-Awsat newspaper that the nuns were safe.
He said they would not be freed until several demands were met.
These include the release of 1,000 Syrian women held in regime prisons according to the spokesman.
The reports have not yet been independently confirmed.
An official at the Greek Orthodox Patriarchate in Damascus said the nuns were safe but would not comment on which group had taken them.
Islamist fighters who captured the Christian village of Maaloula north of Damascus moved the nuns from the Greek Orthodox monastery of Mar Thecla to the nearby town of Yabrud on Monday, according to the Vatican envoy to Syria, Mario Zenari.
The militants took the village after heavy fighting with Syrian army forces in the Qalamoun region near the Lebanese border.
Syria's Christian minority have generally tried to stay on the sidelines of the sectarian conflict involving majority Sunni Muslims against the Alawite minority of President Bashar al-Assad.
Many Christians in Syria fear the rise of hardline Islamist groups.