The leader of the western-backed Syrian opposition coalition has resigned, citing what an insufficient lack of international support for those seeking to topple president Bashar al-Assad.

Moaz al-Khatib, a respected preacher who has led the Syrian National Coalition since soon after its creation in November, said in a statement posted on his Facebook page that he was fulfilling a promise to quit if certain undefined red lines were crossed.

"I am keeping my promise today and announcing my resignation from the National Coalition so that I can work with freedom that is not available inside the official institutions," he wrote.

He also blamed world powers for providing inadequate support to the rebels and complained that many "international and regional parties" insisted on pushing the opposition towards dialogue with the regime. Most opposition leaders and activists say the president's regime has killed too many people to be part of the solution to the conflict.

"All that has happened to the Syrian people – from [the] destruction of infrastructure to the arrest of tens of thousands, to the displacement of hundreds of thousands to other tragedies – is not enough for an international decision to allow the Syrian people to defend themselves," he wrote.

Khatib was chosen to serve as president of the opposition's Syrian National Coalition, which was formed amid international pressure to serve as the opposition's official liaison with other countries and co-ordinate anti-Assad forces.

Despite electing a new, US-educated prime minister to lead a planned interim government last week, the coalition has failed to establish itself as the top rebel authority on the ground in Syria, where hundreds of independent rebel brigades are battling Assad's forces.

The coalition did not immediately respond to the resignation. Khatib's spokesman could not be reached for further comment, but the coalition leader has often used his Facebook page to release statements.

The Syrian government has largely ignored the opposition coalition and says the civil war is an international conspiracy to weaken the country.

The UN says more than 70,000 people have been killed since the crisis began in March 2011.