U.S.-European Union (EU) talks on a trans-Atlantic free-trade zone are moving ahead but still face the difficult challenge of reducing regulatory differences, Europe’s top negotiator said Tuesday.Eight months intonegotiations on the Transatlantic Trade and Investment Partnership (TTIP), Karel de Gucht, the EU trade commissioner, said he and his U.S. counterpart agreed that “things are on track.”“The consequence is that the next phase is going to be harder going. This is where real negotiations begin,” said de Gucht, who spent Monday and Tuesday in talks with U.S. Trade Representative Michael Froman in Washington to assess the TTIP negotiations, aimed at creating what could be the world’s biggest and richest free-trade zone.Teams of experts from both sides have met three times so far this year, and de Gucht said the two sides had identified areas of agreement and differences. One major challenge will be negotiating over issues labor and the environment, where the sides have different standards.“We need to make absolutely sure that trans-Atlantic trade and investment supports, rather than undermines, our high standards on these sustainable development issues. We will not sacrifice them for commercial gain,” de Gucht said.