BEIJING — China on Tuesday publicly rebuffed demands by Sweden for information on the fate of a Swedish citizen who was snatched off a train in China by plainclothes officers despite being under the protection of Swedish diplomats.

The Chinese Foreign Ministry insisted that it had no knowledge of what had happened to the seized man, Gui Minhai. Instead, it issued a warning to all diplomats not to break Chinese law, without making any specific accusations against Sweden.

“This is not a matter that falls under the purview of the Ministry of Foreign Affairs,” a spokeswoman for the ministry, Hua Chunying, told reporters at a regular news briefing in Beijing when asked about Mr. Gui’s seizure from the train. “I don’t understand this specific matter.”

Sweden’s Foreign Ministry, which has twice summoned the Chinese ambassador for meetings, said on Tuesday: “Gui Minhai was seized while in the company of diplomatic personnel who were providing a consular service to a Swedish citizen in need of medical attention. It was entirely in accordance with fundamental international rules that entitle us to give our citizens consular support.”