WASHINGTON, Dec 4 (Reuters) - U.S. government agencies

will hold a classified briefing for all senators on Wednesday on

China's alleged efforts known as Salt Typhoon to burrow deep

into American telecommunications companies and steal data about

U.S. calls, according to officials and a notice seen by Reuters.

The FBI, Director of National Intelligence Avril Haines,

Federal Communications Commission Chair Jessica Rosenworcel, the

National Security Council and the Cybersecurity and

Infrastructure Security Agency are set to take part in the 3

p.m. ET (2000 GMT) closed-door briefing, the officials said.

Separately, a U.S. Senate Commerce subcommittee is planning

to hold a hearing on the Salt Typhoon hacking on Dec. 11,

officials said.

The briefing comes amid growing concern about the size and

scope of the reported Chinese hacking into U.S.

telecommunications networks and questions about when companies

and the government can assure Americans over the matter.

U.S. Representative Jake Auchincloss said last month "Salt

Typhoon is the worst telecom hack in American history, and

demands both a proportionate response to the Chinese Communist

Party and increased accountability for U.S. corporations to

prevent these intrusions."

Incoming FCC Chair Brendan Carr said on Wednesday "the Salt

Typhoon intrusion is a serious and unacceptable risk to our

national security." Carr said he will work "with national

security agencies through the transition and next year in an

effort to root out the threat and secure our networks."

The White House declined to comment. U.S. officials have

previously alleged the hackers targeted Verizon, AT&T ( T )

, T-Mobile, and others and stole telephone audio

intercepts along with a large tranche of call record data.

Chinese officials have previously described the allegations

as disinformation and said Beijing "firmly opposes and combats

cyber attacks and cyber theft in all forms."

CISA told reporters on Tuesday that it could not offer a

timetable for ridding America's telecom networks of all hackers.

"It would be impossible for us to predict when we'll have

full eviction," CISA official Jeff Greene said.