(BRUSSELS) - The United States and European Union agreed on Friday to work "full throttle" to secure the world's biggest-ever free trade deal despite growing scepticism on both sides of the Atlantic.

Negotiators ended an eighth set of talks on the controversial Transatlantic Trade and Investment Partnership, or TTIP, which after nearly two years remains bogged down by activist opposition.

They agreed to hold two more rounds of talks before summer, one of them in April the other later in the first half of the year, chief EU negotiator Ignacio Garcia Bercero said.

"We have received a clear instruction to intensify our talks and to make as much progress as possible this year," he told a news conference in Brussels after five days of talks.

His US counterpart Dan Mullaney added: "We are working full throttle on TTIP, working across the board."

But US Vice President Joe Biden, during a visit to the EU's headquarters in Brussels, underlined the difficulties in overcoming public scepticism about the deal between the world's two biggest economies.

Campaigners in Europe are convinced that powerful interests are selling the consumer short in the secret negotiations, while economic powerhouse Germany is notably tepid on the idea.

"One of the things we have to convince the American people of is that Europe is as interested in this process as we are," Biden told a news conference with EU president Donald Tusk.

"The US and EU are committed to breaking down the remaining barriers to trade that have been holding us back from achieving the full potential of what is already an incredible transatlantic alliance."

The meeting was the first since the new European Commission led by Jean-Claude Juncker took office in November, with the outspoken Swede Cecilia Malmstroem charged with bringing a "fresh start" as the new trade commissioner.

Mullaney said the priority was as great as ever to achieve TTIP, despite criticism that Washington is far more focused on closing a separate deal with Asia.

"This week's round was constructive but we do need to see more concrete progress along the lines that our ministers agreed if we are to turn the fresh start into a reality," Mullaney said.

The EU-US trade deal would not just slash the already low trade tariffs they share but would also harmonise regulations to an unprecedented degree, affecting goods and services as far-ranging as Roquefort cheese and accounting.

But campaigners are opposed to many aspects.

The most contentious part of the deal -- a clause which allows corporations to sue governments in tribunals that are above national law -- would not be on the negotiating table in this round of talks, as parties engage in a "consultative process", Bercero said.