US-EU talks on a transatlantic free trade zone are moving ahead but still face their toughest challenge, cutting back regulatory differences, Europe's top negotiator Karel de Gucht said Tuesday. Eight months into negotiations on the Transatlantic Trade and Investment Partnership (TTIP), de Gucht, the European Union commissioner for trade, said he and his US 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," he said. "If we want to finish on the now-proverbial single tank of gas, our message to our negotiators now is that we need to step up a gear." De Gucht spent Monday and Tuesday in discussions with US Trade Representative Michael Froman in Washington to take stock of the TTIP talks, aimed at creating what could be the world's largest and richest free-trade zone. Teams of experts from both sides have met three times since last year, and de Gucht said the two sides had identified areas of agreement, but more so marked out their differences. "Certainly, the marked-out areas are still larger than the common ground. But we now have a clear picture of the whole field," he said. A key challenge will be the haggling over issues on labor and the environment, where the sides have markedly different standards that could be tested. "We need to make absolutely sure that transatlantic trade and investment supports, rather than undermines, our high standards on these sustainable development issues. We will not sacrifice them for commercial gain," said de Gucht. Most difficult though will be harmonizing regulatory differences that hamper trade, which go well beyond traditional trade issues like tariffs to environmental and health protection and involve issues of biology and chemistry. "It's difficult technically and difficult politically," he said, but on those issues the EU and US "share much more than where we differ." Both de Gucht and Froman said they are committed to fashioning a deal despite the political challenges any pact will face on both sides of the Atlantic. "Our resolve and the political will to reach an ambitious, comprehensive agreement remain strong," said Froman, in a separate statement.