NEW DELHI, April 13 (Reuters) - Big Indian crypto exchanges CoinSwitch Kuber and WazirX have disabled rupee deposits for the purchase of cryptocurrency through a widely-used state-backed transfer network, spurring users to renew demands for regulatory clarity.

India has spent years working on a law to ban or regulate cryptocurrencies, with a ban backed by the central bank over risks to financial stability, though a recent decision to tax income from them suggests acceptance by authorities.

Wednesday's moves follow a one-line statement last week by the National Payments Corporation of India, operator of the state-backed United Payments Interface (UPI), which eases bank transfers, that it was unware of its use by any crypto exchange.

CoinSwitch was not allowing users to load deposits on its app, although they could still withdraw funds.

"You have closed the INR deposit without any information. At least let us know how long it will be closed," a Twitter user, Avijit Debnath, urged the exchange on the social media platform.

CoinSwitch, which says it has more than 15 million users, did not immediately respond to Reuters queries.

The NPCI also did not immediately respond.

An industry source with direct knowledge of the matter said the decision by CoinSwitch to halt UPI acceptance resulted from "regulatory uncertainty" after the NPCI statement.

Rival exchange WazirX also told users on Twitter, "UPI is not available," adding that it had no estimated time limit to fix the issue.

In a statement, WazirX said its deposit facility via UPI had been disabled in December, but declined to elaborate.

Users had expressed concern earlier, when the company said on April 1 that it was disabling rupee deposits via popular payments app MobiKwik "until further notice".

In February, India decided to tax income from cryptocurrencies and other digital assets at 30%, signalling that authorities accepted digital currencies, but uncertainty over regulation has weighed on the industry.

"Regulatory clarity is the need of the hour," said Abhishek Malhotra, a founding partner of TMT Law Practice. "There are currently a lot of conflicting signals on the regulatory regime, leading to lack of certainty."

In October, CoinSwitch said it had raised more than $260 million, for a valuation of $1.9 billion, underscoring the rise in popularity of crypto trading.

No official data is available on the size of India's crypto market, but industry estimates say the number of investors ranges from 15 million to 20 million, with total holdings of about 400 billion rupees ($5.25 billion).