Syria's main opposition leader said he had urged military defectors to limit their actions to defending anti-government protesters but feared his influence may not be enough to prevent civil war.

Syrian National Council leader Burhan Ghalioun said he pressed the leader of the Free Syrian Army, an umbrella group of armed rebels, to rein in operations after they launched a series of attacks on troops loyal to President Bashar al-Assad.

We are worried that we will slide toward a civil war which pits a free army and an official army against each other, he told Reuters in an interview late Thursday. We want to avoid a civil war at all costs.

In the last month army rebels have attacked security forces, destroyed part of an armoured convoy, opened fire on an intelligence centre on the outskirts of Damascus and killed six pilots at an air force base.

Ghalioun said he had asked the head of the Free Syrian Army, Colonel Riyad al-Asaad, to limit his actions to the protection of the demonstrators... but never to launch attacks, operations against the Syrian army forces.

Asaad agreed but insisted that the Free Syrian Army actions were defensive operations, Ghalioun said.

I hope he will keep his word and it is fundamental for the success of our revolution to preserve its peaceful character, that means popular demonstrations, Ghalioun said. We do not want to transform into (a) militia that fights against an army.

The United Nations says at least 4,000 people have been killed in Assad's crackdown on protests which were inspired by uprisings in Tunisia, Egypt and Libya. Authorities blame the violence on armed groups and say 1,100 soldiers and police have been killed since the demonstrations erupted in March.

ASSAD RESPONSIBLE

Ghalioun was in Vienna to rally support for his 260-member opposition council formed in Istanbul three months ago. He said foreign intervention may be inevitable if the bloodshed continued, but Assad would bear responsibility if that happened.

It is up to the current regime to avoid a civil war, meaning an army against an army and military intervention that everyone wants to avoid, he said.

I think that it is not up to us to demand (intervention), but it will happen by itself in any case. If the regime ... continues to kill hundreds a day, the world cannot sit back and do nothing.

Ghalioun addressed around 500 supporters in Vienna on Thursday. Flanked by bodyguards, he emphasised the importance of opposition unity and peaceful demonstrations.

The university professor heads the main opposition National Council but it is unclear what influence he has over protesters inside the country or what role he would play in Syria if Assad were to be toppled.

Asked what he was doing to address the concerns of minority Christians, Alawites and Kurds in a post-Assad Syria, Ghalioun said the population was united and any future government must safeguard equality in the mainly Sunni Muslim state.

After the tensions which the regime has introduced in the population, there needs to be a strengthening of national sentiment by presenting a plan for a democratic, secular state which respects all of its citizens and which criminalises all discrimination -- ethnic or politic or religious, he said.

Ghalioun said he believed Assad would be toppled but it was impossible to predict when. No one can say. I think he has completely lost his legitimacy as president, he said.

I hope that he will stand down by himself.

(Reporting by Sylvia Westall, editing by Dominic Evans and Robert Woodward)