"For this (a decision regarding deliveries of S-300), a key requirement is the withdrawal of a lawsuit. They (Iranians) have not withdrawn it yet, we are waiting. We cannot (simultaneously) take part in legal proceedings over one contract and implement deliveries on the other," Sputnik quoted a high-ranking governmental source as saying on Tuesday.
Under a contract signed in 2007, Russia was required to provide Iran with at least five S-300 defense system batteries.
In 2010, then-president Dmitry Medvedev cancelled the contract as he came under strong US and Israeli pressure not to go ahead with the sale of the weapons system, but his pretext was that the sale was covered by the fourth round of the UN Security Council sanctions against Iran.
Later, Iran lodged a $4 billion lawsuit at an international court in Geneva against Russia's arms export agency.
Back in April, Russian President Vladimir Putin decided to lift the ban on the S-300 missile system delivery to Iran.