Syrian President Bashar al-Assad said his future could only be decided through the ballot box, in an interview with Russian television where he warned the country could face a protracted war.

Assad told state-run Russia Today (RT) that whether the president can "stay or leave" is a "popular issue" and "the only way (it) can be done (is) through the ballot boxes".

"It is not about what we hear. It is about what we can get through that box and that box will tell any president to stay or leave very simply," said the president, speaking in English.

In the interview with a Russia Today correspondent recorded in Damascus, he also denied Syria was in a state of "civil war" but said the conflict with rebels could be "a long-term war" if they continued to receive support from abroad.

Assad described as "unprecedented" the support which he said the rebels were receiving from abroad in terms of arms, money and political backing.

"So, you have to expect that it is going to be a tough war and a difficult war. You do not expect a small country like Syria to defeat all those countries that have been fighting us through proxies just in days or weeks."

If the support for rebels from abroad stopped, he said, "I can tell that in weeks we can finish everything."

"But as long as you have a continuous supply in terrorists, armaments, logistics and everything else, it is going to be a long-term war."

But Assad denied that the country was in civil war as such conflicts should be "based on ethnic problems or sectarian problems".

"You have divisions, but division does not mean civil war," he added.