* U.S. and Turkey hold "expert" talks
* Washington fears impact on allied defences
* Erdogan says NATO concerns not determining factor
By Tulay Karadeniz
ANKARA, Oct 24 (Reuters) - The United States is concerned Turkey's decision to build a missile defence system with a Chinese firm could undermine allied air defences, its envoy said on Thursday, but dismissed talk of a broader rift with Ankara.
U.S. Ambassador to Turkey Francis Ricciardone said Washington had begun "expert" talks with Turkey to assess the impact of its plans to co-produce the long-range air and missile defence system with a Chinese firm under U.S. sanctions.
He downplayed speculation, however, that ties between the two had soured following U.S. newspaper reports in recent weeks suggesting Turkish intelligence chief Hakan Fidan had shared sensitive information with Iran.
Washington sees Turkey as a key partner in the Middle East, with common interests from energy security to counter-terrorism, but Ankara is not the deferential ally it once was as it seeks an increasingly independent role on the world stage.
"We are seriously concerned about what this means for allied missile and air defences for us and for Turkey," Ricciardone said of the Chinese missile defence deal.
"We have really just begun expert discussions with the government of Turkey. We will keep that very respectful, this will be done in official channels as between allies and friends," he told reporters in the capital Ankara.
Turkey, a member of the NATO military alliance, said in September it had chosen the FD-2000 missile defence system from China Precision Machinery Import and Export Corp, or CPMIEC, over rival systems from Russian, U.S. and European firms.
CPMIEC is under U.S. sanctions for violations of the Iran, North Korea and Syria Nonproliferation Act.
NATO Secretary-General Anders Fogh Rasmussen has also voiced concern about Turkey buying a system that is not compatible with those of other members, potentially undermining a core principle of the 28-nation alliance.
Turkey has said it is likely to sign the $3.4 billion deal with CPMIEC but that its decision is not yet final.
Ankara is seeking to bolster its air defences and establish a stronger domestic defence industry, seeing a growing threat of spillover from the war in neighbouring Syria, as well as wider turbulence in the Middle East.
China's foreign ministry has dismissed Western concerns about the plans, saying the United States and others were needlessly politicising a commercial deal.
"STEP TOWARDS THE FUTURE"
Turkish officials also insist the move is not politically motivated and say China's offer met its primary demands on price and the ability to place much of the production in Turkey.
"We are taking a step towards the future and our personnel will be involved in all of the laboratory work," Prime Minister Tayyip Erdogan said on Wednesday, adding that NATO's views on the matter were "not a determining factor".
"If NATO is so sensitive on this subject, many countries which are currently NATO members still have Russian weapons in their inventories," Erdogan told reporters.
Ricciardone said some NATO allies were former Warsaw Pact countries with legacy Soviet systems that were being phased out.
The China deal has sparked speculation of a broader rift between Ankara and Washington.
A report in the Washington Post last week said Turkey had deliberately blown the cover of an Israeli spy ring working inside Iran in early 2012 and dealt a significant blow to Israeli intelligence gathering.
Officials from Erdogan's ruling AK Party have said such accusations are part of a deliberate attempt to discredit both Fidan and Erdogan and to undermine Turkey's role in the region.
"I have had the personal and official privilege of meeting Mr Fidan many times and working with him substantively on issues of common concern," Ricciardone said, adding he had great respect for the Turkish spy chief.
"We have close co-operation with the government of Turkey dealing with the most sensitive problems of the region ... It has been the case for a long time and our alliance is going to continue."