Short-seller Hindenburg Research, which in recent months has taken aim at firms including Nikola Corp and DraftKings Inc, announced a reward of up to $1 million for information on the reserves that back cryptocurrency Tether.

In a statement on Tuesday, the short-seller said Tether's disclosures around its holdings have been "opaque."

The U.S. Commodity Futures Trading Commission fined the company behind Tether $41 million last week to settle civil charges over allegedly making misleading statements and making illegal transactions.

The firm behind the digital currency Tether said in an email it understands its responsibility and will continue to focus on its "customers' requirements and defending the crypto markets."

Tether is a major player in the stablecoin market. Stablecoins are digital tokens usually backed by reserves of dollars or assets.