TAORMINA, Italy (Reuters) - European Council President Donald Tusk urged Group of Seven leaders on Friday to stick to their sanctions policy on Russia over the Ukraine crisis, a day after a senior U.S. official said Washington had no position on the issue.

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The 28-nation EU bloc and the United States imposed sanctions on Russia after Moscow annexed Crimea from Ukraine in 2014 and then backed separatist rebels in eastern Ukraine.

While EU leaders have so far backed sanctions until a shaky ceasefire agreement signed in February 2015 in Minsk is fully implemented, U.S. President Donald Trump’s promise of warmer ties with Moscow has tested the EU’s resolve to remain united.

“A solution to the conflict can only be reached with the full implementation of the Minsk accords,” Tusk said ahead of the summit that gathers the leaders of the United States, Germany, France, Britain, Italy, Japan and Canada.

“Since our last G7 summit in Japan (in 2016) we haven’t seen anything to justify a change in our sanctions policy towards Russia. Therefore I will appeal to the other G7 leaders to reconfirm this policy,” Tusk told reporters in Sicily.

He warned that this summit would be the most challenging in years, given sharp differences on key issues such as climate change and trade.

Tusk was responding to comments by White House economic adviser Gary Cohn on Thursday that appeared to differ to those from U.S. Secretary of State Rex Tillerson, who has repeatedly said sanctions must remain until Minsk is put in place.

“I think the president is looking at it. Right now, we don’t have a position,” Cohn said. Speaking on Friday, Cohn appeared to backtrack on his previous comments.

“We’re not lowering our sanctions on Russia. If anything we would look to get tougher on Russia. The president wants to keep the sanctions in place and I think the president has made it clear how the Russians could have the sanctions lifted.” he told reporters.

COMMON GROUND

Tusk, a former Polish prime minister, said he was optimistic that there would be common ground with Trump on Russia.

“My impression is that when it comes to the conflict in Ukraine we are more or less on the same line as President Trump. Of course. I am maybe less optimistic when it comes to President (Vladimir) Putin’s plans and intentions. I’m less sentimental.”

The G7 nations are due to discuss Russia and the Ukraine crisis on Saturday, a senior French diplomat said.

Fighting between pro-Russian rebels and government forces first broke out in April 2014 after a pro-European uprising in Kiev ousted Ukraine’s Moscow-backed president.

About 10,000 people have been killed in the three-year conflict and concerns are growing that the situation could once again rapidly deteriorate.

The diplomatic stand-off with Russia has dragged relations between Putin and the West to a post-Cold War low.