A 2,400-tonne boring machine is working around the clock more than 1,000 metres below ground on the French side of the border, even as Rome threatens to pull the plug as it reviews the 8.6 billion euros (7.56 billion pounds) project.

The two countries agreed in December to freeze new contracts on the 36-mile (58 km) tunnel until completion of an Italian cost-benefit analysis.

Griveaux added that France had not yet made a decision over whether to challenge the EU's veto to a deal between Siemens and Alstom.

(Reporting by Michel Rose; Editing by Sudip Kar-Gupta and Leigh Thomas)