Syrian President Bashar al-Assad has said Moscow was still committed to sending him advanced anti-aircraft weapons, although a source close to the Russian defence ministry said the missiles had yet to arrive.
Speaking to Hezbollah's al-Manar television, Mr Assad said he would attend talks in Geneva convened by Washington and Moscow, but expected to keep fighting.
By taking part in peace talks, Syria would effectively be negotiating with its international foes who back the opposition, he said: "When we negotiate with the slave we are actually negotiating with the master."
Russia, which has supported Assad's family since the Cold War, says it will send the S-300 missiles in part to help prevent the West from imposing a no-fly zone.
A source close to the Defence Ministry in Moscow said the "hardware itself" had not yet arrived, although the contract was being implemented.
A Lebanese newspaper earlier quoted Assad as saying in his al-Manar interview that Moscow had already sent a first shipment of missiles, although when the actual interview was broadcast Assad appeared to stop short of saying the missiles had arrived.
"Everything we have agreed on with Russia will take place, and part of it has already taken place," he said, without giving further details.
US Senator John McCain has said he is confident the United States can send weapons to fighters in Syria without the risk they will fall into the wrong hands.
"We can identify who these people are. We can help the right people," McCain said on CNN's programme Anderson Cooper 360.
Mr McCain, a Republican, is an outspoken advocate for US military aid to the rebels fighting Syrian President Bashar al-Assad.
He has angrily denounced Democratic President Barack Obama for shying away from deeper US involvement in the conflict, which has claimed 80,000 lives.
Critics of some lawmakers' push to arm the rebels have expressed concerns that weapons could end up in the hands of militants who might eventually end up using them against the United States or its allies.
But Mr McCain said such radical fighters make up only a small part of the rebels forces.
For example, he said, Syria's Islamist al-Nusra Front, identified as an alias of al Qaeda in Iraq, accounts for only about 7,000 of the 100,000 fighters battling the government of Assad.
"Every single day, more and more extremists flow in... They're flowing in all the time, these extremists. But they still do not make up a sizeable portion," the Arizona senator said.
