President Bashar al-Assad has said countries that "sow chaos" in Syria could suffer from it themselves.
In an interview with Russian state news channel Rossia 24, Assad also said that Syrian security forces had captured "foreign mercenaries" who were fighting for the opposition.
Meanwhile, the Britain-based Syrian Observatory for Human Rights said troops stormed the Shammas neighborhood in the central city of Homs overnight, killing at least 15 people.
The watchdog said some of those killed were subjected to "summary executions."
The group updated its toll of people killed in Syria on May 15 to more than 60.
Meanwhile, UN observers were picked up on May 16 by other UN personnel in the town of Khan Sheikhun, one day after a roadside bomb hit their convoy and left them stranded overnight with rebel forces.
None of the observers was injured in the May 15 violence.
Based on reporting by AFP and AP