Syria's opposition on Wednesday denounced what it called "threats" from the head of the Lebanese Shiite group Hezbollah, and warned against any intervention by the movement or by Iran in the Syrian conflict.

"The Syrians and the Lebanese hoped... that the Hezbollah leadership would stop their attacks on Homs and Damascus and take into account the gravity of the situation in the region," the Syrian National Coalition said in a statement.

"But they heard nothing but threats... and warnings against setting the region on fire and an admission of their interference in Syrian affairs," the key opposition grouping said.

On Tuesday evening, Hezbollah chief Hassan Nasrallah acknowledged that members of his group are fighting inside Syria and suggested Iran and other states could intervene to support the Syrian regime against rebel fighters.

President Bashar al-Assad has "true friends in the region who will not allow Syria to fall into the hands of the United States, Israel and 'takfiri' groups," he said.

"If the situation gets more dangerous, states, resistance movements and other forces will be obliged to intervene effectively in the confrontation on the ground," he added.

"You will not be able to bring down the regime militarily," Nasrallah told Syria's rebel forces. "The battle is still long."

The Syrian opposition has long accused Hezbollah of dispatching fighters to battle alongside government forces, and the Coalition called on the Lebanese government "to immediately put an end to Hezbollah military operations in the regions close to the Syrian border."