Syrian leader Bashar al-Assad came under greater pressure amid continuing violence as Russia told him to either reform or resign and Washington said he should "step down now".
Russian President Dmitry Medvedev said Assad will have to leave power if he fails to implement reforms acceptable to the opposition.
"Russia wants as much as the other countries for Syria to end the bloodshed and demands that the Syrian leadership conduct the necessary reforms," Medvedev said in televised remarks.
"If the Syrian leadership is unable to undertake these reforms, it will have to go. But this is something that has to be decided not by NATO or individual European countries but by the people and the leadership of Syria," he said.
Russia vetoed the UN Security Council sanctions resolution against its Soviet-era ally on Tuesday after arguing that moves aimed against the Syrian leadership could encourage protesters to resort to violence.
Medvedev said the most recent decision on Syria was in large part influenced by the NATO-led campaign in Libya that Russia opposed but refused to block at the Security Council in March.
"Russia will continue standing against attempts to legitimise through the UN Security Council unilateral sanctions aimed at toppling various regimes," he said.
"This is not what the UN was created for," said Medvedev.
"In essence, the text that was being proposed was a text that once again permitted the use of force," he stressed.
A senior foreign ministry official said Saturday Russia expects a delegation of Syrian opposition politicians to visit Tuesday.
"We are ready to meet them at the foreign ministry. This could happen on October 11 if they manage to arrive in time," deputy foreign minister Mikhail Bogdanov told the ITAR-TASS news agency.
The delegation, which will meet Russian diplomats on an unofficial basis, will consist of "five or six representatives of various opposition parties," Bogdanov said, speaking on the sidelines of a forum in Rhodes, Greece.
"We are ready to listen to the arguments of the opposition of the Damascus regime and on the other hand to lay out our assessments and forecasts directly without middlemen."
In the ongoing violence Kurdish activist and opposition spokesman Meshaal Tamo, 53, was killed when four masked gunmen stormed his house in Qamishli in the north and opened fire, also wounding his son and another fellow activist in the Kurdish Future Party, activists said.
Assad's regime is escalating its tactics against the opposition with bold, daylight attacks on its leaders, the US State Department charged.
Signup to our newsletter and follow us on Facebook and Twitter!
"This is a clear escalation of regime tactics," State Department spokeswoman Victoria Nuland told reporters, referring to reports of Tamo's murder, as well as the beating on Friday of former MP Riad Seif.
Nuland said both opposition leaders were attacked in broad daylight.
In a statement, White House spokesman Jay Carney condemned the attacks, saying they showed "again that the Assad regime's promises for dialogue and reform are hollow".
"The United States strongly rejects violence directed against peaceful oppositionists wherever it occurs, and stands in solidarity with the courageous people of Syria who deserve their universal rights," Carney said.
"Today's attacks demonstrate the Syrian regime's latest attempts to shut down peaceful opposition inside Syria. President Assad must step down now before taking his country further down this very dangerous path."
France also condemned the Syrian regime's "brutal violence" in its crackdown on the opposition and said it was "shocked" by Tamo's murder.
A foreign ministry statement said the violence "shows that the regime of Bashar al-Assad remains deaf to the appeals of the international community".
The official SANA news agency reported Tamo's "assassination" but gave a different account of his death, saying he was killed "by gunmen in a black car who fired at his car".
Tamo, a member of the newly formed Syrian National Council (SNC) opposition grouping, had been released recently after three and a half years in prison.
SNC spokeswoman Basma Kodmani said the regime had "crossed a new stage in the strategy of repression. All opposition leaders must protect themselves".
Elsewhere, 11 civilians were shot dead in the central city of Homs by security forces, three in Douma and one in Zabadani northwest of Damascus, the Syrian Observatory for Human Rights said.
Another man died after being shot by security forces in the flashpoint northern town of Jisr al-Shughur near the Turkish border, the Local Coordination Committees activist network reported.
Meanwhile, ex-MP Seif had to be given hospital treatment after being beaten outside a mosque in the capital's commercial neighbourhood of Medan.
Mosques in Syria, as happens every week, again became springboards for Friday anti-regime protests, also this time in support of the SNC, formed to represent the main opposition groups, activists said.
Pro-democracy activists had called for fresh demonstrations under the banner: "The Syrian National Council is our representative, mine, yours and that of all Syrians."
Demonstrators in the restive Damascus district of Barzeh carried slogans affirming their "complete support" for the SNC, according to videos on YouTube, while protesters in Homs chanted "the people want the fall of the president".