BEIRUT –  Syria's main opposition groups rejected on Sunday a new international plan that calls for a transitional government because the compromise agreement did not bar President Bashar Assad from participating.

Their reaction held out little hope for an end to more than 15 months of carnage on a day when the main opposition group said 800 people were killed in violence in the past week alone.

Opposition activists groups say more than 14,000 people have been killed since the uprising against President Bashar Assad's authoritarian rule began in March 2011, or on average about 900 a month. That would make last week's toll alone almost as high as the monthly average as government forces furiously pounded rebellious towns and cities with helicopters, tanks and artillery in an offensive aimed at recovering rebel-held territories.

World powers at a conference in Geneva on Saturday accepted a U.N.-brokered plan calling for creation of a transitional national unity government with full executive powers in Syria. But at Russia's insistence, the compromise agreement left the door open to Assad being part of the interim administration. It could also include members of Assad's government and the opposition and other groups. The transitional government would oversee the drafting of a new constitution and elections.

However Syria's fragmented opposition has long opposed any solution that involved negotiating with Assad or allowing him to cling to power.

Bassma Kodmani, a Paris-based spokeswoman for the main opposition group, the Syrian National Council (SNC), said the agreement is "ambiguous" and lacks a mechanism or timetable for implementation. She said there were some positive elements in the plan, which implies that all members of the Security Council were in agreement that the transition period must not be led by Assad. But she said this needs to be more explicit.

"We cannot say that there is any positive outcome today," Kodmani said.

The regime did not react to the plan. But Assad has repeatedly said his government has a responsibility to eliminate terrorists — his term for those fighting the regime — and will not accept any non-Syrian model of governance.

Fayez Sayegh, a prominent lawmaker and member of the ruling Baath party, expressed satisfaction at the outcome of the conference, saying participants left it up to the Syrian people to decide their fate and form of governance.

"The conference ... did not discuss matters that have to do with the president as Western countries would have wished," Sayegh told The Associated Press.

The need for a solution to the Syrian crisis is growing more urgent by the day with the sharp escalation in violence and deaths and the conflict threatening to spill across borders. Syria shot down a warplane from neighboring Turkey on June 22 and Turkey responded by setting up anti-aircraft guns along the frontier. Turkey said Sunday it scrambled fighter jets to its border after Syrian helicopters flew too close to the frontier.

But any hopes for a quick breakthrough were dashed by the opposition's rejection to the brand new initiative, likely relegating it to the latest in a series of failures by the international community to unify and stop Assad's crackdown on dissent.

At the Geneva conference, the U.S. backed away from insisting that the plan should explicitly bar Assad from any role in a new government, hoping the concession would encourage Russia to put greater pressure on its longtime ally to end the violence.

U.S. Secretary of State Hillary Clinton insisted on Saturday that Assad would still have to go and France's foreign minister echoed the demand on Sunday.

It is now "incumbent on Russia and China to show Assad the writing on the wall" and help force his departure," Clinton said, addressing the two countries that have shielded Assad's regime from U.N. Security Council resolutions condemning the crackdown.

The SNC criticized the plan as too ambiguous though it said it contained some new, positive elements. Other opposition groups called it a waste of time and vowed as they always do not to negotiate with Assad or members of his "murderous" regime.

"Every day I ask myself, do they not see how the Syrian people are being slaughtered?" veteran opposition figure Haitham Maleh asked. "It is a catastrophe. The country has been destroyed and they want us then to sit with the killer?"

Maleh described the agreement reached in Geneva as a "farce" and of "no value on the ground."

"The Syrian people are the ones who will decide the battle on the ground, not those sitting in Geneva or New York or anywhere else," he said by telephone from Cairo, where opposition groups are to meet Monday.

The Local Coordination Committees, a key activist network, said it was "very concerned" over the vague language used in the agreement.

"This provides yet another opportunity for the regime's thugs to play their favorite game in utilizing time in order to stop the popular Syrian revolution and extinguish it with violence and massacres," it said in a statement.

The U.N. plan was brokered by special envoy for Syria Kofi Annan, who in March submitted a six-point peace plan that he said the Assad regime accepted. It led to the April 12 ceasefire that failed to hold. U.N. observers sent to monitor the ceasefire suspended their patrols in Syria on June 16 due to a spike in violence and have been confined to their hotels since.

Moscow had refused to back a provision that would call for Assad to step aside, insisting that outsiders cannot order a political solution for Syria and accusing the West of ignoring the darker side of the Syrian opposition. The opposition has made clear it would not take part in a government in which Assad still held power.

Russian Foreign Minister Sergey Lavrov underlined that the plan does not require Assad's ouster, saying there is "no attempt in the document to impose on the Syrian people any type of transitional process."

Even as the international powers met to find a solution, the death toll mounted on Saturday. Activists said dozens were killed and wounded in a powerful explosion Saturday evening that hit a funeral procession in a suburb of the capital Damascus. Details of the blast in Zamalka were still murky on Sunday. But amateur videos showed gruesome images of bodies, some with their limbs torn, lying on the ground as people walked about dazed in a cloud of smoke.

The British-based Syrian Observatory for Human Rights said more than 30 people were killed, while the LCC said it had documented the names of 40 of the dead and that residents were unable to identify an unknown number of remaining bodies.

Activists blamed government forces for the explosion, which they said was likely the result of a car bomb detonated near a mosque where the funeral of an activist killed by regime gunmen was being held.

The SNC did not explain how it arrived at the death toll of 800 for the past week. But it said in a statement that most of the dead were killed in indiscriminate tank and helicopter shelling by regime forces on residential areas throughout Syria.

Death tolls are virtually impossible to verify in tightly-controlled Syria, which imposes severe restrictions on journalists.

But Khalil Al Haj Saleh, a member of the Local Coordination Committees activist network, said the 800 figure appears to be "realistic" in light of the past week's carnage.

The LCC and the Observatory have reported an average of around 100 Syrians killed in the past week.

Turkey scrambled its jets days after it said it would treat any Syrian military unit approaching its border as a direct threat in response to the downing of a Turkish reconnaissance plane by Syrian forces. Turkey has also reinforced its border with anti-aircraft guns and other weapons.

The military said the two helicopters flew as close as 4 miles (6.5 kilometers) to the border province of Hatay on Saturday morning and Saturday afternoon.

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AP writers Albert Aji in Damascus and Suzan Fraser in Ankara contributed to this report.