Russia plans to honour a deal to deliver an advanced surface-to-air missile system to Iran, a senior Russian official said on Sunday according to Interfax news agency.

Vladimir Nazarov, deputy secretary of the Russian security council, said there was no reason not to send Iran the S-300 missile system, in comments reported by the agency.

"A contract was signed which we must fulfill, but deliveries have not started yet. This transaction is not subject to any international sanctions as it only involves the delivery of purely defensive weapons," Nazarov said.

"On the other hand, all of our actions must contribute to global and regional stability, respecting the law and international agreements," he said.

Russia has given vague details on progress in the contract and angry Iranian officials blame the delay on growing pressure from the United States and Iran's arch-foe Israel.

Last week Tehran boasted it was creating its own air defence missile system which would be better than the S-300. However it also threatened legal action in November if Moscow failed to honour the 800 million dollar deal, which involves five batteries of S-300PMU1 missiles.

The S-300PMU1 -- codenamed the SA-20 Gargoyle by NATO -- is a mobile system designed to shoot down aircraft and cruise missiles.

Western governments fear Iran could use the system to boost defences around its nuclear sites against any Israeli or US air strike.

Neither country has ruled out using a military strike on Tehran's nuclear sites to prevent the Islamic republic acquiring an atomic bomb. Iran insists its uranium enrichment project is purely for peaceful purposes.

On Wednesday Russia's Deputy Foreign Minister Sergei Ryabkov said new sanctions on Iran were more likely following Tehran's announcement it had started work to produce 20 percent enriched uranium.

But Nazarov said Sunday care must be taken to avoid sanctions leading to "undesirable consequences".

"If we are talking about sanctions, we must not back Iran into a corner," he said.

Sanctions "must be adequate for the threat posed by the Iranian nuclear programme, but they must not punish the Iranian people," Nazarov said.

Russia has traditionally resisted the threat of tough action, but has recently hardened its stance this week.

Moscow said Iran's move to boost uranium enrichment raised doubts over its claims not to be pursuing atomic weapons.