The federal government on Friday said it is investigating a Chinese hack of the U.S. telecommunications infrastructure, and former President Donald Trump and Republican vice presidential nominee Sen. J.D. Vance are reportedly among those targeted in the operation.

The apparent hack appears to be the latest in a string of operations by U.S. adversaries — namely Russia, Iran and China — aimed at candidates, their campaigns, and America’s election system. The operation, carried out by actors linked to Beijing, may have given China a window into sensitive information contained on the phone of Mr. Trump, a former president with access to highly classified material.

In a joint statement, the FBI and Cybersecurity and Infrastructure Security Agency said the companies affected by the hack have been notified and that anyone who believes their data may have been compromised should contact authorities. The federal agencies did not name Mr. Trump or Mr. Vance in their statement, but media reports Friday said that the two Republican candidates may have had their cell phone data exposed.

“The U.S. government is investigating the unauthorized access to commercial telecommunications infrastructure by actors affiliated with the People’s Republic of China,” the FBI and CISA said in their statement.

“After the FBI identified specific malicious activity targeting the sector, the FBI and the Cybersecurity and Infrastructure Security Agency immediately notified affected companies, rendered technical assistance, and rapidly shared information to assist other potential victims,” the statement said. “The investigation is ongoing, and we encourage any organization that believes it might be a victim to engage its local FBI field office or CISA. Agencies across the U.S. government are collaborating to aggressively mitigate this threat and are coordinating with our industry partners to strengthen cyber defenses across the commercial communications sector.”

The New York Times, citing individuals familiar with the matter, first reported that Mr. Trump’s and Mr. Vance’s phones were targeted. It was not immediately clear what kinds of data, if any, may have been accessed.

Democrats, possibly including individuals inside Vice President Kamala Harris’ presidential campaign, also were reportedly targeted.

Last month, three Iranian operatives were charged with hacking Mr. Trump’s campaign in an operation that began in May, federal authorities said.

Just this week, U.S. intelligence officials said the Russian government created a fake viral video of a person claiming to be a former student of Democratic vice presidential candidate Tim Walz accusing him of sexual abuse.

“The [intelligence community] assesses that Russian influence actors created and amplified content alleging inappropriate activity committed by the Democratic vice presidential candidate during his earlier career,” the Office of the Director of National Intelligence wrote in a foreign election interference update.

Officials warn that the worst may be yet to come.

Senate Intelligence Committee Chairman Mark R. Warner told The Washington Times this week that if the Nov. 5 vote is as close as anticipated, U.S. adversaries can be expected to ramp up digital disinformation operations with the goal of sowing chaos, discord and confusion among Americans in the days immediately following the election.

Mr. Warner told The Washington Times in an interview this week that he believes operations by “China and Iran in particular” will “ramp up as we get closer to Election Day,” and may involve sophisticated deepfake videos.

“The two, three, four days after the election is where we could really see bad things happen,” he said, adding: “Think about the potential of a deepfake of someone who appears to be an election official, [on] Election Day or the day after election appearing to destroy ballots, and artificial intelligence can just simply allow these activities to be done at a speed and scale that’s unprecedented.”