The outsiders are from Iran, Yemen, Iraq and the Lebanese Shiite movement Hezbollah, Haithem al-Mallah, leader of the opposition and Nation Coalition, told the Kuwaiti daily Alrai Monday.
"The Syrian forces of [President] Bashar Assad cannot fight on the ground ... the war on the ground is carried out by the Iranians, Hezbollah, Yemenis and Iraqis," he said.
The Syrian Observatory for Human Rights said there was fighting Monday in Homs, Aleppo, Daraa and Damascus .
The Syrian air force renewed its bombardment on Homs, the group's website said.
The official state news agency SANA said rebel forces, which it referred to as "armed terrorist groups," shot down a helicopter over Nubl city Sunday, killing seven people.
The agency said Assad issued a new law barring children under 18 from being recruited into combat activities. The penalty for people caught recruiting children is 10 to 20 years imprisonment, and if a child dies in combat those responsible for the recruitment will be sentenced to death, the agency said.