April 22 (Reuters) - UnitedHealth Group ( UNH ) said on

Monday the cyberattack at its technology unit led to a breach of

health and personal information, which could cover a

'substantial proportion' of Americans.

The company, however, did not disclose the number of people

impacted by the February cyberattack at the Change Healthcare

division.

It said it would take several months before enough

information to identify impacted individuals.

The company was monitoring the internet and dark web to

determine if data has been published, along with external

industry experts.

With about one in three U.S. patient records accessed by the

unit's health technology offerings, the cyberattack sent

shockwaves across the nation's healthcare system.

The hack at Change, a provider of healthcare billing and

data systems and a key node in the U.S. healthcare system,

disrupted payments to doctors and healthcare facilities

nationwide.

The unit was breached on Feb. 21 by a hacking group called

ALPHV, also known as "BlackCat".