(Adds Turkish comment, context)
LONDON, Aug 16 (Reuters) - A Russian state news agency said on Tuesday that Russia and Turkey had signed a contract to ship Ankara a second batch of S-400 air defence systems, but a Turkish defence official immediately cast doubt on the report.
TASS news agency cited the head of Russia's military cooperation service, Dmitry Shugayev, as saying: "I want to note that the relevant contract has already been signed. It envisions the localisation of production of certain components of the system."
Turkey's initial 2020 purchase of the Russian S-400 system angered the United States, a NATO ally, which imposed sanctions on Turkey in retaliation.
A Turkish defence official said on Tuesday there were "no new agreements".
"The original contract that was signed with Russia for the purchase of S400s already included two batches. The purchase of a second batch was included in the original plan and the related contract," the official said.
"Therefore, we do not have any concrete developments worth reporting. The process is ongoing and there are no new agreements."
Any new defence contract between Russia and Turkey would provoke serious concern from Washington at a time when it is trying to isolate Russia and maintain NATO unity in the face of Russia's invasion of Ukraine.
Turkey and Russia signed a first accord in December 2017 on deliveries of Russian S-400 surface-to-air missile batteries, reportedly worth around $2.5 billion. Turkey received the first deliveries in July 2019.
In response, the United States imposed sanctions on Turkey's defence industry in December 2020 and expelled Ankara from the F-35 stealth fighter jet programme, where it was a manufacturer and buyer. Ankara says the moves were unjust.
Turkey has not joined in Western sanctions against Russia but has been active in mediation efforts since Russia invaded Ukraine in February. It was instrumental in brokering an agreement last month to enable a resumption of Ukrainian grain exports via the Black Sea. (Additional reporting by Ece Toksabay, editing by Mark Trevelyan and Alex Richardson)