WASHINGTON, May 1 (Reuters) -

Members of the U.S. military have had their data stolen in

the

recent cyberattack

on UnitedHealth's ( UNH ) technology unit that impacted

almost all patients and providers, CEO Andrew Witty told the

Senate Finance Committee on Wednesday.

"We do believe there will be members of the armed

forces" caught up in the hack, Witty said in the first of two

scheduled testimonies in front of Congressional panels.

It would take longer than a week to provide lawmakers

with the number of military personnel affected, Witty told the

committee Chairman, Senator Ron Wyden.

The hack at Change Healthcare, a provider of healthcare

billing and data systems that processes about 50% of U.S.

medical claims, on Feb. 21 caused widespread disruption in

payment to doctors and health facilities.

"I believe the bigger the company, the bigger the

responsibility to protect its systems from hackers. UHG was a

big target long before it was hacked. The FBI says that

healthcare industry is the number one target of ransomware,"

Wyden said in opening remarks, adding that the hack was a

national security threat.

Health and personal data of a "substantial proportion" of

Americans were stolen, UnitedHealth ( UNH ) said last week.

"UnitedHealth Group ( UNH ) has not revealed how many patients'

private medical records were stolen, how many providers went

without reimbursement, and how many seniors are unable to pick

up their prescriptions as a result of the hack," said Wyden.

"The failures of CEOs like Mr. Witty, who months in can't

figure out how many people have had their data stolen, validates

the FBI warning," he said.

In letters to the congressional committees Witty will face,

the American Hospital Association said an internally survey of

its members found that 94% of hospitals reported damage to cash

flow and more than half reported "significant or serious"

financial damage due to Change's inability to process claims.

Hackers breached UnitedHealth's ( UNH ) tech unit on Feb. 12 by

using stolen login credentials that gave them remote access to

its network, Witty will tell the Senate Finance Committee on

Wednesday morning and the House Energy Subcommittee on Oversight

and Investigations in the afternoon.

In a written testimony published earlier this week, Witty

said UnitedHealth ( UNH ), the largest U.S. health insurer, has been

working with the FBI and prominent cybersecurity firms to

investigate the hack.