"We respect all our contracts and are honouring all our contractual obligations," Lavrov said in an interview with state-run Rossiya 24 TV. "As yet, the contracts are not finished, they have not been delivered in full," he added.
Russia last month acknowledged it has agreed to sell Syria advanced S-300 air-defence missiles, which are considered to be the cutting edge in aircraft interception technology.
Russia has stood by Syrian President Bashar Assad during the two-year civil war, blocking several U.N. resolutions and calls for his ouster. The death toll from the conflict is at least 93,000.
Lavrov said that Russia would block all demands for the Assad government's resignation at a planned peace conference to be held in Geneva. He said those calls were a "direct affront to us and the Americans" from unnamed Gulf states.
No date has been set for the conference, which Lavrov blamed on Western countries' fears they would not be able to "prevail upon" the Syrian opposition to attend.
Lavrov renewed Russia's objections to a potential no-fly zone in Syria, which the S-300 systems would make extremely difficult to implement, and said Western plans to arm the opposition would see most weapons fall into the hands of Jabhat al-Nusra, an al-Qaida affiliate that is believed to be among the most effective rebel factions.