Syrian opposition activists gathered in Istanbul to endorse a blueprint for political change intended to unify their movement, but the conference proved divisive even before its official start on Tuesday.

The planned agreement, to be signed Tuesday, follows a flurry of efforts by the opposition Syrian National Council to win international confidence for efforts to oust President Bashar al- Assad. Dissidents are trying to present a more united face ahead of a meeting Sunday of the Friends of Syria, a contact group that brings together representatives from 70 countries and the United Nations.

A rival to the SNC boycotted the meeting, while other opposition groups complained that hosts Turkey and Qatar were undermining the work of a separate U.N.-Arab League effort to end the crisis.

That initiative, led by envoy Kofi Annan, has moved on a separate track from the Friends of Syria, which was formed to work around the U.N. where Russia and China were blocking moves against the Assad regime.

Mr. Annan´s initiative is focused on negotiating with Syria´s government on a six-point plan to end the violence. The plan doesn´t address a call adopted by the first Friends of Syria meeting for Mr. Assad to step aside.

The Annan plan was endorsed by the U.N. Security Council last week with the consent of Russia and China. Mr. Annan is studying a Syrian response to the plan, and will answer shortly, said Mr. Annan´s spokesman, Ahmad Fawzi. Mr. Annan will be represented by his deputy at the Friends of Syria meeting, Mr. Fawzi said.

The Syrian regime´s violent crackdown on opposition has lasted over a year, and left more than 9,000 dead according to the U.N.—the longest and deadliest confrontation in the Arab Spring of pro-democracy uprisings.

Mr. Annan on Monday raised the possibility of U.N. observers entering Syria to enforce an agreement. He said that he hoped the hostilities could cease very shortly, and in that case, "We will have to be on the ground to monitor and ensure that all sides are respecting the agreement."

Mr. Annan was speaking in Moscow after meetings with Russian President Dmitry Medvedev and Foreign Minister Sergei Lavrov, before departing for China. "I had very strong support here in Moscow" for the six-point plan and expects similar in China, he said.

Meanwhile, the Syrian National Council, which was formed in September, is seeking recognition as the sole representative of the opposition and, eventually, the only legitimate representative of the Syrian people. The SNC was recognized in February as a leading representative of the Syrian people at the first Friends of Syria meeting, and the U.S., EU, and some Arab states have for months dealt with the group as the main opposition vehicle.

But the National Coordination Body for Democratic Change—the second- broadest opposition coalition—on Monday said it wouldn´t attend the meeting in Istanbul, calling it "an effort to politically manipulate the Syrian opposition and to cram it under one ceiling of options."

The SNC´s ability to demonstrate unity and win popular support is necessary for international states to move forward in their efforts to aid the opposition, officials said.

"We are giving them a firm, stable consistent message that they have to take on board everybody so that they represent every walk of society in Syria," a Turkish official said.

Arab and Western officials characterized the SNC´s blueprint, known as the "National Covenant for a New Syria," as an effort to focus on forging a unity of vision, rather than structure, in the opposition.

The documentfocuses on free and fair elections in a transitional period, the separation of powers, an independent judiciary, and minority rights. Syria´s ethnic Kurds are wrangling to be named in the pact, opposition members said.

Turkey´s efforts to inject fresh momentum into shoring up the opposition underlines how Ankara, an erstwhile ally of Mr. Assad, has become frustrated with the situation in neighboring Syria. Ankara on Monday said it closed its embassy in Damascus, but is keeping its consulate in Aleppo open.

Turkey has regeared its policy to bolster the SNC, analysts said. "The fear now is that the SNC is losing strength and becoming more fragmented," said Sinan Ulgen, a former Turkish diplomat now at the Carnegie Endowment for International Peace. "Ankara and its allies are trying to essentially help SNC to unite and not to fragment."

Turkey and the U.S. said Sunday they are considering providing communications equipment to help Syria´s opposition and rebel fighters coordinate. President Barack Obama and Turkish Prime Minister Recep Tayyip Erdogan, speaking after a meeting in Seoul, said the Friends of Syria meeting in Istanbul would discuss other nonlethal aid the U.S. could supply.

—Carol E. Lee and Christopher Rhoads contributed to this article (Copyright © Dow Jones & Company, Inc.) 03/27/12)