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SHANGHAI, Feb 5 (Reuters) - China's securities regulator said on Monday it will closely monitor and take forceful measures to prevent risks from pledged shares as the stock market plunged to five-year lows.

The slump has led to many big shareholders of listed companies who borrowed against their stocks facing margin calls.

Roughly 100 listed companies so far this year have disclosed that major shareholders are putting up additional collateral to avoid forced liquidation.

"Meanwhile, we are guiding brokers to increase the flexibility of liquidation levels, to promote the smooth operation of the market," the China Securities Regulatory Commission (CSRC) said.

The CSRC also said the proportion of value of pledged shares have dropped from 10.51% of the total market value at the peak in 2018 to 3.38% so far. (Reporting by Shanghai Newsroom; Editing by Kim Coghill and Tom Hogue)