The head of the Arab League called Monday for the fragmented Syrian opposition to unite and said a U.N.-brokered plan for a transitional government in Syria fell short of expectations.

Speaking at the start of a two-day conference that brought together some 250 members of the Syrian opposition, Arab League Secretary General Nabil Elaraby urged them not to waste the chance presented by the meeting to overcome their differences and band together to help lift Syria out of its crisis.

"There is an opportunity before the conference of Syrian opposition today that must be seized, and I say and repeat that this opportunity must not be wasted under any circumstance," he said. "The sacrifices of the Syrian people are bigger than us and more valuable than any narrow differences or factional disputes."

More than one year into the Syrian revolt, the opposition is still hobbled by infighting, although in general the disparate groups agree that President Bashar Assad should have no role in a transitional period. One main sticking point is how to achieve a peace plan that would end the bloodshed and Assad's authoritarian rule. While some activists have called for international intervention in Syria, others have rejected the idea.

The meeting in Cairo brought together a host of opposition factions _ including members of the Syrian National Council and the Local Coordination Committees _ to try to agree on a united body to represent them, as well as to work out a plan for ending a conflict that activists say has killed at least 14,000 people since March 2011.

"We hope to walk out with one voice to have a national agreement with outlines of a transitional plan that will draw the way forward," said Reema Flaihan, the head of the planning committee for the conference.

The main rebel group fighting Syrian government forces on the ground, the Free Syrian Army, was not represented at the talks. Faiz Amru, a member of the Joint Military Command, which is affiliated with the FSA, said the Cairo meeting was purely political, so rebels were not invited.

Turkey, which has been among the Syrian regime's harshest critics, sent Foreign Minister Ahmed Davutoglu to the talks.

"What we now have in front of us is a regime that has strayed so far away from a basic stance of rationality," he said. "Your only interlocutor as representatives is the Syrian opposition and nobody else."

The Arab League-led meeting follows an international conference of world powers over the weekend in Geneva that accepted Annan's new plan to form a transitional government in Syria to end the country's crisis. But at Russia's insistence, it left the door open to Assad being a part of an interim transition.

Syrian opposition groups have roundly rejected Annan's plan, calling it ambiguous and a waste of time and vowing not to negotiate with Assad or members of his regime.

Elaraby, who has held private meetings with Syrian opposition figures at the League's headquarters in the past, also criticized the plan Monday, saying it fell short of Arab expectations by not specifying a time frame for a "clear transition" as the Arab League had called for.

The U.S. backed away from insisting that the plan should explicitly call for Assad to have no role in a new Syrian government, hoping the concession would encourage Russia to put greater pressure on its longtime ally to end the violent crackdown.

The meeting takes place after one of the uprising's bloodiest weeks, according to the SNC, which says around 800 people were killed in the past week alone.