The Internal Revenue Service is on the war path against Americans who haven’t reported income from cryptocurrencies like bitcoin.

In late July, the IRS said it had started to send warning letters to more than 10,000 people who may not have complied with tax rules on virtual currencies. Agency officials have said criminal tax indictments involving cryptocurrencies are expected soon, and other enforcement letters are going out.

Tax specialists are urging crypto users who aren’t in compliance to act quickly. While coming clean involves a maze of tricky decisions, ignoring the agency could cost a crypto holder dearly. ...

The IRS is focusing on cryptocurrencies because their use is expanding, and enthusiasts often praise the anonymity virtual currencies offer. Many trades aren’t reportable to the IRS by third parties—unlike sales of securities such as stock shares, which generally must be reported to the IRS by brokerage firms. An IRS analysis found that for 2013, 2014, and 2015, when more than 80% of returns were electronically filed, fewer than 1,000 e-filed returns each year reported transactions appearing to use virtual currencies. Coinbase said at the end of 2013 that it had 650,000 accounts. Now it has more than 30 million.

Cryptocurrency advocates are upset by the IRS’s campaign. They say the agency hasn’t yet released long-promised guidance, including how to pinpoint some fair market values; which cost-allocation methods to use; or whether the agency favors a small exemption for personal use. Recently, 60% of global bitcoin transactions were below $600, according to Coin Metrics, a cryptocurrency data provider. Bloomberg.