Deputy Foreign Minister Sergei Ryabkov also accused the European Union of "throwing fuel on the fire" by letting its arms embargo on Syrian expire, saying it would complicate efforts to arrange an international peace conference.
His remarks toughened Russia's defiance of the United States, France and Israel over the planned sale of precision S-300 missile systems to President Bashar al-Assad's government, which is battling a Western and Gulf Arab-backed insurgency.
"We think this delivery is a stabilising factor and that such steps in many ways restrain some hotheads ... from exploring scenarios in which this conflict could be given an international character with participation of outside forces, to whom this idea is not foreign," he told a news conference.
Western experts say the air defence system could significantly boost Syria's ability to stave off outside intervention in the more than two-year civil war that has killed more than 80,000 people.
The S-300s can intercept manned aircraft and guided missiles and their delivery would improve Assad's government's chances of holding out in Damascus. Western nations say the Russian arms deliveries could increase tension and encourage Assad.
Moscow is standing firm on the sale, despite a trip to Russia by Israeli Prime Minister Benjamin Netanyahu this month in which he pleaded with President Vladimir Putin to halt the delivery, and a veiled warning of a military response by Israel.
"I can say that the shipments are not on their way yet," Israeli Defence Minister Moshe Yaalon said on Tuesday at a conference near Tel Aviv. "I hope they will not leave, and if, God forbid, they reach Syria, we will know what to do."
Russia has sent anti-missile defence systems to Syria before, but says it has not sent offensive weapons or arms that can be used against the anti-government forces. A source close to Russia's state arms exporter said a contract to supply Syria with fighter jets had been suspended.
Ryabkov was unable to confirm whether S-300s had already been delivered but said "we will not disavow them".
Russia has been Assad's most powerful ally during the conflict, opposing sanctions and blocking, with China, three Western-backed U.N. Security Council resolutions meant to pressure the government to stop fighting.
Moscow opposes military intervention or arming Syrian rebels and defends its right to deliver arms to Assad's government.
Ryabkov said the failure by the EU to renew its arms embargo on Syria at a meeting on Monday would undermine the chances for peace talks which Moscow and Washington are trying to organise.
"The European Union is essentially throwing fuel on the fire in Syria," he said of the EU compromise decision which will allow EU states to supply arms to the rebels if they wish.
His comments were echoed by Putin's press secretary, Dmitry Peskov, who also criticised a visit to Syria on Monday by U.S. Senator John McCain, who met rebels fighting Assad's government.
Britain and France, which opposed renewing the arms embargo, have made clear they reserve the right the send arms immediately, despite an agreement by European countries to put off potential deliveries until August 1, but have made no decisions yet.
A senior French official said the S-300 was brought up at talks between French Foreign Minister Laurent Fabius, Russian Foreign Minister Sergei Lavrov and U.S. Secretary of State John Kerry in Paris on Monday.
"Obviously it poses a huge problem for us because if they deliver these weapons - they are ground-to-air missiles - and if we were to set up air corridors, then you can see the contradiction between the two," the official said.
Israel says Russian weapons sent to Syria could end up in the hands of its enemy, Iran, or the Lebanese Hezbollah group.
Israeli Strategic Affairs and Intelligence Minister Yuval Steinitz said the S-300 could reach deep into the Jewish state and threaten flights over its main commercial airport near Tel Aviv.