BRUSSELS (Reuters) - European Union leaders aim to send a "clear signal" to U.S. President-elect Donald Trump at a summit on Thursday about their continued support for Ukraine and will also discuss the security and economic challenges posed by his return to the White House.

The leaders will host Ukrainian President Volodymyr Zelenskiy at the start of their talks in Brussels and will reaffirm their "unwavering commitment" to supporting Ukraine "for as long as it takes", according to draft conclusions.

Trump has repeatedly called for a swift end to the nearly three-year-old war. On Monday he said Zelenskiy should be ready to reach a peace deal with Russian President Vladimir Putin, though he did not say whether this meant Kyiv ceding territory to Moscow as part of a negotiated settlement.

Russian forces currently occupy nearly a fifth of Ukrainian territory and are making steady advances in the east of the country.

"Russia must not prevail," the EU draft conclusions say, adding that no initiative must be taken on Ukraine without Kyiv's involvement.

One EU diplomat described the draft text as sending "a clear signal to the U.S.".

The leaders will also discuss wider EU-U.S. relations over lunch amid concerns of a possible transatlantic trade war.

Trump has said the EU will "pay a big price" with tariffs for not buying enough U.S. exports. He has already pledged hefty tariffs on three of the United States' largest trading partners - Canada, Mexico and China. The EU knows it will not be spared.

UNITY

Some EU diplomats said the key for the bloc would be unity and avoidance of Washington conducting discussions or deals with single EU members - a copy of its largely successful unified strategy for dealing with Britain during Brexit negotiations.

"The U.S. might try individual negotiations but so far I have not seen countries falling for that," one EU diplomat said.

The EU will seek to point out that it is the United States' second-biggest trading partner and a close ally with shared values. However, mindful that Trump is preoccupied by the U.S. goods trade deficit, EU officials have mooted potentially offering to buy more U.S. LNG or arms.

The lunchtime "EU in the world" debate is also likely to include China and whether the bloc will be forced to take sides in a U.S.-China trade conflict as well as Britain, which has said it wants to reset ties with the European Union and is seen by the bloc as a crucial security ally.