Mounting pressure from central bank authorities has forced leading bitcoin exchanges in China to suspend withdrawals of the cryptocurrency.

Huobi and OKCoin announced last Thursday that it is completely stopping bitcoin and litecoin withdrawals for one month, effective immediately. In a post, the two Chinese digital currency exchanges said halting the withdrawal feature was part of the decision to improve their anti-money laundering capabilities and prevent “illegal transactions.”

OKCoin said only its OKCoin.cn portal is affected by the suspension.

During the one-month period, OKCoin and Huobi would go through an “upgrade” to fight “money laundering, exchange, pyramid schemes and other illegal activities.” BTC China also announced that it is subjecting all bitcoin withdrawals to a 72-hour review as it upgrades its internal systems.