WASHINGTON — The top Democrat on the Senate Foreign Relations Committee is requesting a State Department investigation into the use of personal cellphones by senior U.S. diplomats in the wake of revelations in the impeachment hearings that have raised profound concerns about potential cybersecurity breaches.

Sen. Robert Menendez of New Jersey wrote this week to Michael Evanoff, the assistant secretary of state responsible for diplomatic security, asking that senior officials be punished if allegations are borne out showing they conducted “sensitive national security business” via unsecure personal devices.

The request comes as the ambassador to the E.U., Gordon Sondland, a major GOP donor and Trump political appointee, testifies Wednesday in a televised hearing about a phone call he had with President Donald Trump while Sondland was in Kyiv. Several others present overheard the conversation, in which they discussed Ukraine’s willingness to acquiesce to Trump’s request that it open investigations into his political opponents, other witnesses have testified.

“Given the importance of this issue and the apparent culture of indifference to classification matters and information security emanating from President Trump, I request your office immediately undertake a review of communication security at the Department,” Menendez wrote in the letter, which was obtained by NBC News.

He added, “I assume you share my alarm.”

Menendez said the investigation should cover the use of personal cellphones “by all political appointees” at the State Department. He also asked the agency to “assess any security breaches, incidents or infractions, make recommendations, and take appropriate disciplinary action.”

The State Department and its diplomatic security bureau didn’t respond to a request for comment. Menendez, in his letter, asked that Evanoff respond by Dec. 2.

Sondland’s call with Trump was just the latest troubling disclosure during the impeachment inquiry that has raised concerns about how top Trump administration diplomats are communicating.

Text messages turned over to Congress showed Sondland, former Ukraine envoy Kurt Volker and others communicating with one another and with Ukrainian officials about official government business via text message and the messaging app WhatsApp, raising concerns not only about cybersecurity but also about compliance with federal record-keeping requirements.

Although it’s unclear whether Sondland used his personal cellphone during the overheard call with the president on July 26, Fiona Hill, the former top White House official for Europe, has testified that Sondland “was using his own personal cellphone at all times.” She told impeachment investigators she “became extremely concerned that his communications were not going to be secure.”

Hill also told the House that Sondland had given her own personal cellphone number out to foreign governments. She testified that Sondland’s indiscreet use of cellphones to discuss national security matters constituted a "counterintelligence risk.”

And David Holmes, the U.S. diplomat who testified he was with Sondland when he called Trump from a Kyiv restaurant, said Trump’s voice was so loud that he could hear the president’s side of the conversation for part of the call, even as waiters came and went. Asked if there was a risk that Russia was listening in on the call, Holmes agreed.

“I believe at least two of the three, if not all three, of the mobile networks are owned by Russian companies, or have significant stakes in those,” Holmes said. “We generally assume that mobile communications in Ukraine are being monitored.”

That concern is not unfounded. In 2014, a recording of then-Assistant Secretary of State Victoria Nuland disparaging the European Union in a phone call with the U.S. ambassador in Ukraine was leaked, an embarrassing incident that the State Department had to clean up. The U.S. accused Russian spy agencies of tapping the call.