The U.N. observers also visited a cemetery where some of the dead were buried, according to an activist in Mazraat al-Qubair.

Activists said the Sunni hamlet is surrounded by Alawite villages. Alawites are an offshoot of Shiite Islam and Assad is a member of the sect, while the opposition is dominated by Sunnis.

The United States condemned Assad over the killings, saying he has "doubled down on his brutality and duplicity."

The violence followed another mass killing last month in a string of villages known as Houla, where 100 people including many women and children were also shot and stabbed to death. The opposition and the regime blamed each other for the Houla massacre.

In April, the U.N. said more than 9,000 people have been killed since the crisis began in March 2011, but it has been unable to update its estimate since and the daily bloodshed has continued in past weeks. Activists put the number of dead at about 13,000.