Deadly violence raged across Syria on Thursday with a twin bombing in Damascus two days before a crucial international meeting that looked threatened late in the day by Russian objections.
Turkey reinforced its volatile border with Syria with missile batteries as world powers prepared for talks in Geneva on ways to end the raging conflict and to discuss a plan by peace envoy Kofi Annan for an interim government.
But UN-Arab League envoy Annan was battling to save the meeting following Russian objections to his proposed transition plan, diplomats said at the United Nations.
Violence on Thursday killed at least 91 people, including 59 civilians, after one of the bloodiest days of the 15-month revolt left at least 149 dead on Wednesday, a watchdog said.
Thursday's heaviest toll was in the northern Damascus suburb of Douma where 30 people were killed, among them five members of one family, said the Syrian Observatory for Human Rights.
The day's fatalities also included 23 soldiers and nine rebels, said the campaign group, adding that regime forces backed by helicopters pounded several areas of the eastern city of Deir Ezzor.
On the diplomatic front Annan called talks between senior officials from Russia, the United States, France, Britain and China on Friday in a bid to rescue the meeting of foreign ministers the next day, diplomats said.
A meeting between US Secretary of State Hillary Clinton and Russian Foreign Minister Sergei Lavrov in Saint Petersburg on Friday could also decide the fate of future international action on Syria.
Clinton and the foreign ministers of Britain and France, William Hague and Laurent Fabius, have told Annan there will be no point holding the Geneva meeting unless an accord on a transition plan can be guaranteed, diplomats said.
The Geneva conference with Clinton, Lavrov, Hague, Fabius, China's Foreign Minister Yang Jiechi and the foreign ministers of Qatar, Turkey and Kuwait had been intended as a public show of support for Annan's transition plan.
Annan announced the meeting on Tuesday, having said he would only call the ministers to Geneva if he was sure they would unite around a plan to end the worsening conflict that Syrian activists say has left more than 15,000 dead.
Russia, the last major ally of President Bashar al-Assad, has objected to a proposal which could limit membership of a transitional unity government in Syria, diplomats said.
Annan's plan, obtained by AFP, said the interim government could include Assad officials and the opposition "but would exclude from government those whose continued presence and participation would undermine the credibility of the transition and jeopardise stability and reconciliation".
Diplomats have said this means that Assad could be ruled out of the government but did not automatically exclude his participation. Opposition figures could also be kept out under the same formula, they stressed.
The meeting in Geneva, agreed only after wrangling between Moscow and Washington over the agenda and the guest list, was to be attended by some regional governments but not Middle East heavyweights Iran and Saudi Arabia.
Russia insists Iran should be part of the solution to Syria's conflict. "Iran is an influential player in this situation and to leave it out of the Geneva meeting, I believe, is a mistake," Lavrov said on Thursday.
Assad's fate "must be decided within the framework of a Syrian dialogue by the Syrian people themselves" without foreign interference, he insisted.
Russia had already poured cold water on the Geneva gathering, with Lavrov saying Moscow rejected Western pressure for Assad's ouster.
In central Damascus, three people were wounded when bombs blasted a car park outside the Palace of Justice complex, state media reported. A police source told AFP that two magnetic bombs exploded in judges' cars.
US State Department spokeswoman Victoria Nuland said Washington condemned attacks "against innocents from any direction that they come from" after the bombings.
"But the larger concern is -- as we've said for many, many weeks -- it is Assad that bears the brunt of responsibility," she told reporters.
More than 15,800 people have been killed since the uprising broke out in March 2011, including nearly 4,700 since April 12, when a UN-backed ceasefire was supposed to have taken effect, the Observatory says.
Turkey has sent missile batteries, tanks and troops to the border as a "security corridor" after Syria shot down a Turkish warplane last Friday, media reports said.
State-run TRT television showed dozens of military vehicles reportedly heading for the border, in a convoy that included air defence systems.
About 30 military vehicles accompanied by a truck towing missile batteries left a base in the southeastern province of Hatay for the frontier, about 50 kilometres (30 miles) away, Milliyet newspaper reported.
The Turkish Phantom F-4 jet was downed by Syria over the eastern Mediterranean in what Prime Minister Recep Tayyip Erdogan called a "heinous attack" over international waters.
Turkey's National Security Council said after a meeting on Thursday it will take steps "with determination by reserving all of its rights born out of international law in the face of this hostile act."