Russia has started the delivery of the advanced S-300 air defense system to Iran, despite American and Israeli objections.

“The contract is in action. They’ve begun,” Russian presidential aide for military-technical cooperation Vladimir Kozhin said, the Russian news agency TASS reported.

The S-300 surface-to-air-missile was initially developed under the Soviet Union, and its newer variants are regarded as the most potent anti-aircraft missile systems in the world.

Initially signed in 2007, the $800 million deal between Iran and Russia for the S-300 system was suspended in 2010. But in April, Russian President Vladimir Putin lifted the suspension after the framework nuclear deal between Iran and the P5+1 powers. A Russian official in early November, several months after the final nuclear deal was reached, confirmed that the sale was moving forward.

The move comes despite Israeli-Russian military coordination over the Syrian civil war in order to avoid inadvertent clashes such as the recent incident in which a Turkish fighter jet shot down a Russian fighter jet.

The U.S., Israel, and Saudi Arabia have all opposed the sale of the S-300 system to Iran. Israel and the U.S. fear that the missile defense system could make it more difficult to use the military option against Iran’s nuclear program.