The 28-point peace plan was crafted by the Trump administration and the Kremlin without Ukraine’s involvement. It acquiesces to many Russian demands that Ukrainian President Volodymyr Zelenskyy has categorically rejected on dozens of occasions, including giving up large pieces of territory. Trump says he wants Ukraine to accept the plan by late next week.

The senators’ opposition to the plan follows criticism from other U.S. lawmakers, including some Republicans, none of whom have the power to block it. The senators, who spoke at an international security conference in Canada, included a Democrat, an Independent and a Republican who does not plan to seek reelection next year.

“It rewards aggression. This is pure and simple. There’s no ethical, legal, moral, political justification for Russia claiming eastern Ukraine,” Independent Maine Sen. Angus King said during a panel discussion at the Halifax International Security Forum in Canada.

King, a member of the Senate Foreign Relations Committee, compared the proposal to British Prime Minister Neville Chamberlain’s Munich Pact with Adolf Hitler in 1938, a historic failed act of appeasement.

Republican Sen. Thom Tillis of North Carolina said Sen. Mitch McConnell, a former Republican Senate party leader, didn’t go far enough in his criticism of it. McConnell said in a statement Friday that “if Administration officials are more concerned with appeasing Putin than securing real peace, then the President ought to find new advisers.”

“We should not do anything that makes (Putin) feel like he has a win here. Honestly, I think what Mitch said was short of what should be said,” said Tillis. Tillis announced earlier this year that he would not seek reelection shortly after he clashed with the Trump administration over its tax and spending package.

Democratic Sen. Jeanne Shaheen of New Hampshire, the ranking Member of the Senate Foreign Relations Committee, called it an “outrage.”

Putin welcomed the proposal late Friday, saying it “could form the basis of a final peace settlement” if the U.S. can get Ukraine and its European allies to agree.

Zelenskyy, in an address, did not reject the plan outright, but insisted on fair treatment while pledging to “work calmly” with Washington and other partners in what he called “truly one of the most difficult moments in our history.”

In its 17th year, about 300 people gather annually at the Halifax International Security Forum held at Halifax’s Westin hotel. The forum attracts military officials, U.S. senators, diplomats and scholars but this year the Trump administration suspended participation of U.S. defense officials in events by think tanks, including the Halifax International Security Forum.

A large number of U.S. senators made the trip this year in part because of strained relations between Canada and the U.S. Trump has alienated America’s neighbor with his trade war and insistence that Canada should become the 51st U.S. state. Many Canadians now refuse to travel to the U.S. and border states like Shaheen’s New Hampshire are seeing a dramatic drop in tourism.

“There’s real concern about that strain. That’s one reason why there’s such a big delegation is here,” Shaheen said. “I will continue to object to what the president is doing in terms about tariffs and his comments because they are not only detrimental to Canada and our relationship, but I think they are detrimental globally. They show a lack of respect of sovereign nations.”