Russia says Syria needs a period of political transition but has rejected calls for President Bashar al-Assad to relinquish power in a possible unity government. Meanwhile, there has been a powerful explosion near the country's main court in Damascus.

Thursday's developments came as neighboring Turkey deployed troops and anti-aircraft batteries to the Syrian border following the downing of one of its military jets by Syrian forces last week.

In Moscow, Foreign Minister Sergei Lavrov said any solution to the crisis in Syria must be decided by Syrians themselves and that Russia would not support external “meddling.”

Syrian opposition groups said Thursday they would not accept any proposed political transition plan that does not explicitly require Mr. Assad to step down.

The diplomatic wrangling emerged ahead of Saturday's planned “action group” meeting in Geneva where international envoy Kofi Annan could propose a national unity cabinet that includes opposition figures. Diplomatic sources at the United Nations said the plan does not demand Mr. Assad's resignation but excludes those whose participation could undermine stability. Details were vague.

U.S. Secretary of State Hillary Clinton told reporters in Latvia that Annan's plan calls for “a Syrian-led transition.” She said Washington believes any solution must comply “with international standards on human rights, accountable governance, the rule of law and equal opportunity for all people of Syria. Clinton said the Annan framework “lays out how to arrive at that.”

Lavrov and Clinton are due to meet Friday in St. Petersburg.

Joshua Landis, a Syria expert at the University of Oklahoma, told VOA the Obama administration has steadily gained confidence that its policy of regime change “is the correct one and is going to happen sooner or later.” He said Russia still appears convinced it can find a way to keep Assad loyalists in power even without the Assad family itself.

But Landis said the balance of power in Syria is changing.

“The Arab majority, the Sunni Arab majority, is going to win this in the long run. That's what's been happening throughout the Middle East in the last several decades.”

Landis predicted the transition from minority Alawite domination to Sunni Muslim rule in Syria would be protracted and messy. But he noted that Western and Arab sanctions on the Syrian government and assistance to the rebels are already bearing fruit.

“Western Europe, the Gulf countries, America are starving the Syrian government with very strict sanctions. And they are feeding the opposition, pumping in money, arms [and] intelligence. This is rapidly changing the balance of power.”

Although Mr. Assad's government still has many assets, Landis said, the rebel Free Syrian Army “is becoming much more lethal, getting much better at terrorist-type attacks and is taking the fight to the regime.”

Syrian state television called Thursday's explosion near the Palace of Justice in central Damascus a “terrorist” attack. Dozens of burned out cars were strewn around a car park used by lawyers and judges. Three people were reportedly wounded.

The FSA has stepped up pressure on the capital in recent weeks.

Meanwhile, Turkish military convoys moved toward the Syrian frontier, reacting to Syria's downing of a Turkish warplane on Friday. The deployment came two days after Prime Minister Recep Tayyip Erdogan ordered his troops to treat any Syrian military element approaching the border as a target.

Turkey hosts over 33,000 Syrian refugees on its southeastern border as well as FSA units fighting to overthrow Mr. Assad.

Mr. Annan's transition proposal is one of the main topics that five permanent U.N. Security Council members and some key players in the Middle East will discuss on Saturday.

Lavrov said Moscow has not agreed to any new version of Mr. Annan's crumbling cease-fire proposal and reiterated his government's strong opposition to any outside efforts that would force regime change in Syria.

“The meeting in Geneva was intended to support Kofi Annan's plan and it must set the conditions for the end of violence and the start of an all-Syrian national dialogue, and not pre-determine the contents of this dialogue.”

The Russian foreign minister said it was a “mistake” not to invite Syrian ally Iran to the Geneva meeting, calling the country an “influential player” in the situation. Saudi Arabia, a prominent supporter of those opposed to Mr. Assad, was also left off the list.