Crypto miners' bitcoin reserves slid to the lowest point since May following a spate of withdrawals this week, in a sign of mounting selling pressure as the largest cryptocurrency posts a December gain of more than 13%, data from CryptoQuant shows.

Miner reserves measure the number of coins held by the affiliated miners' wallets. The number drops as the coins are moved to crypto exchanges, possibly as a prelude to a sale.

Miners began balancing their books in late October, with reserves entering a decline that accelerated this month. Reserves now number 1.832 million BTC being held by miners, down from October's high of 1.845 million.

In a post on social-media platform X, AliCharts pointed out that miners have sold 3,000 bitcoin (BTC) over the past 24 hours, equating to around $129 million. The cryptocurrency is currently trading at $42,891, down from yesterday's high of $43,710.

The net flow of bitcoin on Dec. 28 stood at minus 1,524 BTC, meaning that withdrawals exceeded new coins being minted, data shows.

Bitcoin will undergo a halving in April, which will see miner rewards slashed to 3.125 BTC per block from 6.25 BTC. Analysts predict that the halving will cause a supply shock with bitcoin potentially reaching $160,000.

Read more: Bitcoin Halving Is Coming and Only the Most Efficient Miners Will Survive