WASHINGTON — A potentially explosive complaint by a whistle-blower in the intelligence community said to involve President Trump emerged on Thursday as the latest front in a continuing oversight dispute between administration officials and House Democrats.

While the allegation remains shrouded in mystery, it involves at least one instance of Mr. Trump making an unspecified commitment to a foreign leader and includes other actions, according to interviews. At least part of the allegation deals with Ukraine, two people familiar with it said.

The complaint, submitted by a member of the intelligence community to its inspector general, renewed questions about how the president handles delicate matters. Mr. Trump defended his actions, and allies described his style with foreign leaders as more freewheeling than typical high-level diplomacy. “I would only do what is right anyway, and only do good for the USA!” Mr. Trump wrote on Twitter.

Though it is not clear how Ukraine fits into the allegation, questions have already emerged about Mr. Trump’s dealings with its government. In late July, he told the country’s new president, Volodymyr Zelensky, that Ukraine could improve its reputation and its “interaction” with the United States by investigating corruption, according to a Ukrainian government summary of the call. Some of Mr. Trump’s close allies were also urging the Ukrainian government to investigate matters that could hurt the president’s political rivals, including former Vice President Joseph R. Biden Jr. and his family.