A Lebanese security source said the nuns had been taken to the Lebanese town of Arsal earlier in the week and were headed to Syria on Sunday accompanied by the head of a Lebanese security agency and a Qatari intelligence official.
After being held in the Greek Orthodox monastery of Mar Thecla in Maaloula, they were reportedly moved to the rebel-held town of Yabroud, about 13 miles to the north, which is now the focus of a government military operation.
Shortly after the nuns disappeared, Islamist rebels said they had taken them as their "guests" and that they would release them soon.
The Observatory and a rebel source in the area said the release of the nuns had been agreed as part of a swap in which the government would free scores of women prisoners.
"The deal is for the release of 138 women from Assad's prisons," the rebel source said, referring to Syrian President Bashar al-Assad.
In December, the nuns appeared in a video obtained by Al Jazeera television, saying they were in good health, but it was not clear under what conditions the video had been filmed.
Syrian state television devoted significant coverage to the release on Sunday, but made no mention of any prisoner exchange agreement. It broadcast live footage from the Lebanese border and interviews with church officials, including one who denounced the West as only believing "in the dollar".
A montage of Christian religious imagery including churches, a statue of the Virgin Mary and murals of Jesus was set against dramatic music and described Syria as a "cradle of the monotheistic faiths."
Syria's Christian minority has broadly tried to stay on the sidelines of the three-year-old-conflict, which has killed over 140,000 people and which has become increasingly sectarian.
But the rise of hardline Islamists among the overwhelmingly Sunni Muslim opposition has alarmed many. Assad, whose minority Alawite sect is an offshoot of Shi'ite Islam, has portrayed himself as a bulwark against militant and intolerant ideologies.