By Julia Payne and Jan Strupczewski

BRUSSELS (Reuters) -The European Union has changed its team locked in trade talks with the Trump administration, bringing in a close aide to European Commission President Ursula von der Leyen to deal more swiftly with political questions arising from the technical negotiations, three sources familiar with the talks said.

The move follows frequent outbursts of frustration by U.S. President Donald Trump at what the White House perceives as slow progress in talks with the EU. Late in May he recommended a 50% tariff on most European goods from June, before backtracking.

The move to bring more political decision-making to the EU team reflects challenges confronting the bloc as it negotiates a trading relationship with a U.S. president who has said repeatedly the EU was established to screw the United States.

It also reflects the difficulties negotiating trade terms in isolation when Trump has sought to fold non-tariff barriers such as digital services taxes and food standards into the talks.

"If you are a trade negotiator you need to be sure you have full political backing, so if the top level is there you feel stronger," one of the sources said.

The expanded team, which apart from the von der Leyen aide now also includes a cabinet member of Trade Commissioner Maros Sefcovic, was dispatched to Washington this week after a call between Trump and von der Leyen in which they agreed to fast-track negotiations.

Following that call Trump agreed to allow more time for talks between Washington and the 27-nation bloc to produce a deal by July 9.

"It was a merger of Commission layers to reinforce and act fast," a second of the sources said of the team’s expansion, adding the team’s shape could change again as talks continue.

U.S. Trade Representative Jamieson Greer said a meeting with Sefcovic in Paris on Wednesday had been constructive and that he was pleased negotiations were advancing quickly. He noted "a willingness by the EU to work with us to find a concrete way forward to achieve reciprocal trade".

Sefcovic told reporters that both sides had concluded talks were "advancing in the right direction, at pace," and that high-level contacts would follow shortly. He and Greer had agreed how to "restructure" the focus of negotiations with the United States, he added.

Washington was focused on four areas in its negotiations with other countries: tariffs, non-tariff barriers, purchases and economic security, one of the sources said.

Trump has already hit Europe with a 50% tariff on steel and aluminium as well as a heightened levy on car imports. The EU is racing to secure a deal before July 9 when "reciprocal" tariffs on most other goods could surge from 10% to as high as 50%.

Unlike Britain, the first major economy to reach a narrow trade agreement with the Trump administration, the EU is pushing for a comprehensive deal, with a baseline tariff rate below the 10% now in force.

Two of the three sources said additional technical expertise had also been added to the negotiating team, but there had been no changes to the team’s leadership.

"I think it suits everyone to have the political cover," the first source said.