The leader of the opposition Syrian National Coalition (SNC), Ahmed Moaz al-Khatib, has resigned.
Mr Khatib, who had been in the job barely four months, says world powers have failed to help the rebels and he can only improve the situation by working outside the coalition.
Analysts say Mr Khatib believes the coalition is too heavily influenced by Islamists and foreign powers like Qatar.
The BBC's correspondent in neighbouring Lebanon says the issue that brought matters to a head appears to have been last week's election of a prime minister for an interim opposition government.
The SNC chose US-based Islamist Ghassan Hitto to head the alternative administration, which is intended to govern rebel-held areas inside Syria.
Described by some coalition leaders as a "transparent, democratic" election, it also prompted several resignations from the SNC.
The military wing of the opposition, the Free Syrian Army, has also said it will not recognise Mr Hitto because he has been forced on the opposition.
In his resignation statement Mr Khatib said: "For the past two years, we have been slaughtered by an unprecedentedly vicious regime, while the world has looked on.
"All the destruction of Syria's infrastructure, the detention of tens of thousands of people, the forced flight of hundreds of thousands and other forms of suffering have been insufficient for the international community to take a decision to allow the people to defend themselves."