By Tom Fairless and Zeke Turner 

BERLIN -- Germany's Vice Chancellor Sigmar Gabriel warned that negotiations to build a vast trans-Atlantic free-trade zone have failed, casting fresh doubt on a sweeping deal that has faced growing criticism as anti-globalization sentiment gained traction on both sides of the Atlantic.

"From where I'm sitting, the negotiations with the Americans have failed. Just nobody says that," Mr. Gabriel said Sunday during an appearance at a summer open-house event organized by the German government.

The EU and U.S. have been rushing to complete negotiations for the world's largest free-trade zone before the Obama administration leaves office in January. The deal, which would encompass around 800 million consumers, aims to eliminate most tariffs and harmonize regulations in areas ranging from environmental rules to food safety.

Brussels and Washington have been negotiating the Trans-Atlantic Trade and Investment Partnership, or TTIP, since 2013, and both sides still say they hope to conclude it by year-end.

But the deal has faced an unprecedented backlash in Germany, where hundreds of thousands of people have taken to the streets across the country to protest. Opponents, which include trade unions and environmental groups, say the agreement is anti-democratic and could harm environmental and food safety standards.

Meanwhile Britain's vote to leave the EU removes one of the bloc's strongest trade advocates in Europe, as the U.K. frequently acts as a counterweight to some of the bloc's more protectionist countries.

Members of Mr. Gabriel's center-left party spoke out last month to say the negotiations were dead in the water. France's Prime Minister Manuel Valls warned earlier this year that the proposed pact would serve as "a breeding ground for populism."

Chancellor Angela Merkel, a full-throated advocate of the deal from the start, has said she would like the negotiations to be completed before the Germany's federal elections next year.

But those elections, as well as French presidential elections in May, are set to complicate the progress of the deal further. Britain's expected departure from the EU also makes a deal less interesting for the U.S. given the U.K. is America's largest trading partner in Europe, as measured by the total export value for goods, according to the U.S. Department of Commerce.

Longstanding sticking points in the negotiations include European rules protecting regional food names such as feta and Parma ham, and U.S. rules protecting domestic companies bidding for government contracts.

Mr. Gabriel indicated he doesn't oppose free-trade deals per se. He defended a planned deal with Canada, the Comprehensive Economic and Trade Agreement, which he described as "a real quantum leap." That deal would make trade easier without reducing standards for labor and the environment, Mr. Gabriel said.

Mr. Gabriel's Social Democratic Party is set to vote next month on whether to support the Canadian free-trade deal.

However both U.S. presidential contenders have indicated they are skeptical of sweeping free-trade agreements. Hillary Clinton and Donald Trump have criticized the recently agreed-upon Trans-Pacific Partnership, a trade pact between the U.S., Japan, Canada, Australia and eight other countries around the Pacific.

Meanwhile the TTIP deal has been rapidly losing popularity with the German public. In May, public broadcaster ARD's DeutschlandTrend survey showed that 70% of Germans opposed the deal, a 15-point increase since 2014.

Viktoria Dendrinou contributed to this article.

Write to Tom Fairless at tom.fairless@wsj.com

(END) Dow Jones Newswires

August 28, 2016 17:36 ET (21:36 GMT)