Russia announced Monday that it has agreed to push on with a deal to deliver S-300 missile systems to Iran after Moscow halted an earlier agreement due to UN sanctions slapped on Tehran.

"The contract between Russia and Iran for delivery of S-300 missile systems is back in force," the state-run Russian Technologies corporation said in a statement, adding that the two sides had "signed a contract".

Moscow in April lifted a ban on selling the missile systems to Iran, ahead of Tehran sealing a final historic deal with world powers in July to curb its nuclear programme.

The decision sparked condemnation from Israel and concern from Washington, as it came before the lifting of the sanctions by the UN Security Council.

Russia argues that the missile system is exclusively defensive and does not even fall under the sanctions.

Moscow blocked deliveries of the surface-to-air missiles to Tehran in 2010 after the UN Security Council imposed the curbs on Iran over its nuclear programme barring hi-tech weapons sales.

Iran then filed a $4-billion suit at an arbitration court in Geneva for the cancellation of the $800 million order by Russia, which has long been Iran's principal foreign arms supplier.

Moscow says its has now offered Tehran more modern versions of the weapons but has yet to announce publicly when they will be delivered.

Russian Technologies said Iran had agreed to drop its legal claims against Moscow after the "first part" of the contract is fulfilled.

Russia and Iran are allied over their backing for Syrian President Bashar al-Assad, with both sides helping regime forces fighting on the ground.