US arms makers Raytheon Co and Lockheed Martin Corp are considering ways to sweeten their offer to build a Patriot missile defense system for Turkey after Ankara said it could still back away from a provisional $3.4 billion deal with China, sources familiar with the issue said on Tuesday.

“There are internal discussions going on about improving the Patriot offer,” said one source, who is familiar with the talks but not authorized to speak publicly.

A second source confirmed that preliminary discussions were under way within industry and the US government about how the offer could be adjusted to be more competitive with bids submitted by the Chinese firm, and a European group.