The New York Attorney-General's office says it has opened a criminal investigation into former US president Donald Trump's company, going beyond what began as a civil probe. 

Attorney-General Letitia James has been investigating whether the Trump Organization falsely reported property values to secure loans and obtain economic and tax benefits.

"We have informed the Trump Organization that our investigation into the organisation is no longer purely civil in nature," Fabien Levy, spokesman for the Attorney-General's office, said in a statement.

"We are now actively investigating the Trump Organization in a criminal capacity, along with the Manhattan DA (District Attorney)."

The office didn't specify what prompted the change in its approach to the investigation or why it chose to announce it publicly.

The Trump Organization could not immediately be reached for comment.

Ms James's disclosure of a widening investigation is not necessarily an indication that she is planning to bring criminal charges.

In New York, if that were to happen, the state attorney-general can do so through a county district attorney or with a referral from Governor Andrew Cuomo or a state agency.

Mr Trump has previously said that the investigation overseen by Ms James, a Democrat, was politically motivated.

Ms James has been investigating whether the Trump Organization inflated the values of some properties to obtain better loans, and lowered their values to get property tax breaks.

Separately, Manhattan District Attorney Cyrus Vance has been investigating Mr Trump's pre-presidency business dealings for more than two years.

Mr Vance's office said in court filings it was investigating "possibly extensive and protracted criminal conduct" at the former president's Trump Organization, including tax and insurance fraud and falsification of business records.

Mr Vance's probe began after Mr Trump's former lawyer and fixer, Michael Cohen, paid hush money to silence two women before the 2016 election about claimed sexual encounters with the former president.

That investigation has accelerated since Mr Trump lost his bid for a second term to current US President Joe Biden.

Mr Vance's investigation has also appeared to focus in recent weeks on the Trump Organization's long-time finance chief, Allen Weisselberg.

His former daughter-in-law, Jen Weisselberg, has given investigators reams of documents as they look into how some Trump employees were compensated with apartments or school tuition.

Ms James's existing civil investigation and Mr Vance's criminal probe had overlapped in some areas, including examining whether Mr Trump or his businesses manipulated the value of assets — inflating them in some cases and minimising them in others — to gain favourable loan terms and tax benefits.

Mr Vance's investigation included a look at the propriety of tax write-offs the Trump Organization claimed on millions of dollars in consulting fees it paid, including money that went to Mr Trump's daughter, Ivanka.

ABC/wires