Even as Europeans sought to display a united front in their support of Ukraine at a meeting of defense ministers in Brussels on Thursday, concerns are mounting that they could be left out in the cold by a Trump administration push to work with Russia to end the war.

European leaders and top officials have insisted that Ukraine must have a seat at the table as peace plans are drawn and that the process should not be rushed because that could leave the embattled nation in a place of weakness during negotiations.

But there has been a growing concern among NATO allies that the United States will take a different approach — even as it is prodding Europeans to shoulder more of the bill for whatever peace plan comes to pass.

President Trump announced on Wednesday that he had a “lengthy and highly productive phone call” with President Vladimir V. Putin of Russia. He characterized it as the beginning of a negotiation to end the war in Ukraine without mentioning a role for Europe or Ukraine’s president, Volodymyr Zelensky, with whom he had a call as well. That, paired with comments from Mr. Trump and his defense secretary painting key Ukrainian goals as unrealistic, has helped to stoke fears that the United States could fail to take European concerns into account in its push for an end to the conflict.

“There will be no credible and successful negotiations, no lasting peace, without Ukraine and without the E.U.,” said Antonio Costa, the president of the European Council, part of the European Union’s government.

“Peace cannot be a simple cease-fire,” Mr. Costa wrote in a social media post on Thursday afternoon, and “Russia must no longer be a threat to Ukraine, to Europe, to international security.”

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