U.S. President Donald Trump on Friday said he knew nothing about son-in-law and White House adviser Jared Kushner's use of the WhatsApp encrypted messaging tool, a day after a top U.S. Democratic congressman questioned the unofficial communications.

On Thursday, U.S. House of Representatives Oversight Committee Chairman Elijah Cummings asked the White House about Kushner's use of the unofficial messaging application as part of his government work.

In a letter to the White House, seen by Reuters, Cummings said Kushner's lawyer had told lawmakers about his WhatsApp use for official duties, a move that would violate current law prohibiting White House officials from using non-official electronic messaging accounts.

Trump, speaking to reporters at the White House before departing for Mar-A-Lago, his private club in Florida, for the weekend, denied any knowledge of Kushner's unofficial communications.

"I know nothing about it. I've never heard that, I've never heard about it," the Republican president said.

Cummings in his letter on Thursday said Kushner lawyer Abbe Lowell also told Congress that Ivanka Trump — the president's daughter, Kushner's wife and also a top White House adviser — continued to use a personal email account for official business.

That would also violate the Presidential Records Act.

'Not completely accurate'

Lowell, in a separate letter to Cummings, called the Democratic committee chairman's characterization of earlier comments "not completely accurate."

The lawyer denied telling Congress members Kushner had communicated through any app with foreign "leaders" or "officials" but said that instead Kushner had used such apps for communicating with "some people," whom he did not specify.

Lowell also denied saying that Ivanka Trump continued to receive emails related to official business on a personal account. He said Ivanka Trump "always forwards official business to her White House account."

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In the 2016 presidential race, Trump railed against his Democratic opponent, Hillary Clinton, for her use of a private email server while serving as secretary of state, inspiring chants at his rallies of "lock her up." The FBI and the Department of Justice investigated Clinton but brought no charges.

Kushner's communications, particularly with foreign leaders, have been under scrutiny since the presidential campaign, and questions have been raised about his security clearance. WhatsApp is owned by Facebook Inc. (VOA)