A number of "unsolved problems" remain in the negotiations on reforms demanded by Britain ahead of its in-out referendum, EU President Donald Tusk said Tuesday, as he wrapped up a whirlwind European tour meant to win over other member states.

"I have no doubts: there is an extra mile we will have to walk to reach an agreement," he said in Prague after meeting Czech Prime Minister Bohuslav Sobotka. "There are still unsolved problems."

EU leaders will attempt later this week to strike a deal on the reforms, which British Prime Minister David Cameron has said would allow him to campaign for his country to remain an EU member. He has promised to hold a referendum on the matter by the end of 2017.

Fears are rife that Britons might vote to leave the 28-country European Union, destabilizing the bloc politically and economically - and at worst starting to tear it apart.

European Commission President Jean-Claude Juncker, who met with Cameron in Brussels on Tuesday, told parliamentarians that it was "too early to say" if a breakthrough will be achieved. But he insisted that there is no plan prepared other than Britain remaining in the bloc as "a constructive and active member."

In a bid to iron out remaining differences on the reforms, Tusk held talks with European leaders in Paris, Bucharest and Athens before arriving in Prague on Tuesday. He was also due to meet with German Chancellor Angela Merkel in Berlin later in the evening.

"We have to move forward together," Greek Prime Minister Alexis Tsipras said in Athens. "This is not the time for exits [from the EU], this is rather the time for us to join our hands and forces so that we may face all the huge global and regional challenges."

The EU, however, will not "compromise on our freedoms and values," Tusk later warned in Prague.

He underlined that London's bid to restrict social benefits for workers from other EU member states remains among "the most sensitive" issues for central European countries. Many of their citizens live and work elsewhere in the EU.

There are concerns in other EU member states too. France for instance is wary of London's quest to prevent decisions taken by the 19-country eurozone from harming its interests.

"We have to ensure ... that nothing prevents the countries of the eurozone from continuing their advancement and their integration," French State Secretary Harlem Desir said at talks in Brussels to prepare the leaders' summit on Thursday and Friday.

"We do not want to take the path of disintegration," German State Minister Michael Roth added.

Also on Tuesday, Cameron met with European Parliament leaders in Brussels. The premier received assurances that the parliament would act "swiftly" on legislation to implement any agreed deal, a British government spokesperson said.

But Parliament President Martin Schulz said that he could not give any "guarantee" about the outcome of the democratic process. The legislature's ranks include eurosceptic politicians who could attempt to scupper a deal.

"There are many groups here who are spoiling for a fight," EU parliamentarian Nigel Farage, who also leads the eurosceptic UK Independence Party, warned in a statement.

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