The Observatory said gunmen from Fua refused to state the bus's make and color — "an issue that raises suspicion," it said.
The Associated Press was unable to reach residents of the Shiite villages. A message sent by a reporter to a Facebook page for Fua received an unnamed response.
"We will liberate our prisoners by force and we seek help from no one," it said. "We know how to fight, be victorious and take revenge."
It was unclear who wrote the message.
What is clear is that most Sunni activists and rebels expect an armed showdown between the two communities. Some say they see no place for Shiites in the area's future.
"We have warned them that everything in Fua is a target and that everyone who wants to save himself from the problems should leave," said activist Maher Abdel-Ghani, who often works with Islamic extremist rebel battalions.
"God willing, we'll soon liberate the whole area and there will be nothing left called Fua or Kifarya," he said. "Those two villages will be erased because they have harmed us a lot."
Also Saturday, a power outage plunged Damascus and much of southern Syria into darkness, the state news agency reported, quoting Electricity Minister Imad Khamis.
The blackout affected Syria's capital and the southern provinces of Daraa and Sweida, which abut the Jordanian border.
An Associated Press reporter in Damascus reported dark streets across the capital. A fuel shortage makes it hard for residents to run backup generators.
A similar blackout struck the same areas on Jan. 20. The government blamed that outage on a rebel attack, and power was restored to most areas the following day.
The Syrian capital's 2.5 million residents have grown used to frequent power cuts as the country's conflict has damaged infrastructure and sapped the government's finances.
The U.N. says nearly 70,000 people have been killed in Syria's conflict since March 2011.
Associated Press writer Albert Aji contributed reporting from Damascus, Syria.
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