Turkey has asked the United States to extend the pricing on Raytheon Co's Patriot missile defense system proposal, two sources familiar with the discussions told Reuters on Monday, a sign that Ankara is keeping its options open in case its talks with the preferred Chinese supplier fall through.

Turkish Prime Minister Tayyip Erdogan last week said Ankara would be open to new offers if its current talks about co-producing a long-range air and missile defense system with China Precision Machinery Import and Export Corp (CPMIEC) fail.

Turkey's decision to choose a $3.4 billion Chinese offer over rival bids from Russian, US and European firms has raised concern among Turkey's Western allies.

The sources familiar with the US proposal to supply a Raytheon-built Patriot missile defense system said Turkish officials had requested an extension of the pricing included in the bid while their talks continued with China.

"It's clear that they are trying to hedge their bets," said one of the sources, who was not authorized to speak publicly. It was not immediately clear how long of an extension was requested.

Turkey announced in September it had chosen China's FD-2000 missile defense system over rival systems from Franco/Italian Eurosam SAMP/T and Raytheon. It said CPMIEC offered the most competitive terms and would allow co-production in Turkey.

The US ambassador to Turkey said on Thursday that Washington was concerned that the deal with the Chinese firm could undermine allied air defenses and had begun "expert" talks with Ankara to assess the potential impact.

CPMIEC is under US sanctions for violations of the Iran, North Korea and Syria Nonproliferation Act.

NATO is also worried about Turkey buying a system not compatible with those of other member states, potentially undermining a core principle of the 28-nation alliance.

The sources said Turkey's missile defense deal could also affect its plans to buy radar-evading F-35 fighter jets built by Lockheed Martin Corp, which also builds the PAC-3 missiles used by the Patriot missile defense system.