Iran has not received Russia's advanced S-300 anti-aircraft system as the two countries continue discussions on the purchase, say Israeli defence sources, revising earlier statements that a deal was imminent.
The missile system would help Iran fend off any Israeli or United States air strike against its nuclear facilities.
Analysts believe a purchase of the system could accelerate the countdown to military action designed to destroy Iran's nuclear facilities.
"We know that, as of now, nothing has been shipped," an Israeli defence source said on Sunday. "There seems to be some kind of hold-up. The Russians and Iranians are discussing this, but we have also been speaking to Russia about our concerns."
Israeli Prime Minister Ehud Olmert will fly to Moscow on Monday for talks in which he is expected to ask Russia to curb defence sales to Iran and Syria.
Russia has denied intending to sell Iran the S-300, the best version of which can track 100 targets and fire on planes 120 kilometres. The system is known in the West as the SA-20.
Another Israeli source says Iran appears to be vacillating on whether to buy the newest version of the S-300 or a less advanced model.
"Delivery schedule will greatly depend on which version they eventually settle on. If the new one, then it's years away. The other version is readily available," the source said.
Israel, which has the Middle East's only nuclear arsenal, describes Iran's nuclear programme as a potential threat to its existence.
Iran says its nuclear work is a peaceful project to generate electricity.