By Philip Blenkinsop and Alastair Macdonald

BRUSSELS (Reuters) - The European Union summoned envoys from its member states to an emergency meeting in Brussels at 8 p.m. (1800 GMT) on Thursday to seek a last-minute resolution to Belgian difficulties holding up a trade deal with Canada, diplomats said.

All 28 EU governments support the planned Comprehensive Economic and Trade Agreement (CETA), but Belgium cannot give its assent without backing from five sub-federal administrations and the Wallonia region has steadfastly opposed it.

The move came as the 28 national leaders began a two-day EU summit in Brussels and diplomats there said the meeting could be "good news" for supporters of the treaty. Martin Schulz, the president of the European Parliament, told a news conference at the summit: "There could be an agreement within the next hours."

The CETA pact is set to be signed in just a week at an EU-Canada summit next Thursday to be attended by Canadian Prime Minister Justin Trudeau but has looked to be in jeopardy.

A spokesman for Wallonia's regional premier denied a local media report that he was heading to talks with EU officials.

Several EU diplomats said they hoped that Wallonia would accept new concessions on offer to unblock the deal.

Among the new elements on offer, one EU diplomat said, were changes in the text of an EU declaration to be appended to the treaty. These would answer Walloon concerns about agricultural trade and on how trade disputes with Canada would be settled.

European Council President Donald Tusk, chairing the summit, said in a tweet that he was deeply concerned by the CETA difficulties and had held a meeting before it began with Belgian Prime Minister Charles Michel, who strongly backs the accord.

"Still waiting for an answer," Tusk tweeted. "Credibility of Europe at stake."

TRADING FUTURE?

Tusk earlier warned the issue was greater than just a trade deal with Canada, EU's 12th largest trading partner, although the agreement would be the EU's first with a G7 country.

He told reporters there would be no public support for free trade if people could not be convinced that trade agreements were in their interests or that trade negotiators were working for them: "Which means, I am afraid, that CETA could be our last free trade agreement."

Failure to strike a deal with such a like-minded country as Canada would call into question the EU's ability to forge other deals and damage credibility already battered by Britain's vote to leave the bloc and arguments over the migration crisis.

Belgian premier Michel talked on Thursday of a "moment of truth" approaching, adding that the Walloon government had received the final texts on CETA from the European Commission.

"These are difficult moments, delicate moments," he said. "We are close to the moment of truth. At a certain moment in a negotiation you have to say yes or no."

A liberal centrist, Michel faces opposition from Socialist-led Wallonia. Its lawmakers share concerns voiced by many on the European left that CETA, and a stalled plan for a similar and much bigger deal with the United States known as TTIP, risk watering down consumer, labour and environmental protections.

(Additional reporting by Francesco Guarascio and Andreas Kroener; Writing by Philip Blenkinsop; Editing by Alastair Macdonald)