US crude oil prices continued to fall Monday, diving below $40 a barrel to their lowest level since 2009, amid a global market selloff sparked by fears of China's slowdown.

US benchmark West Texas Intermediate for October delivery tumbled by $1.39 to $39.06 a barrel on the New York Mercantile Exchange around 1305 GMT. On Friday the contract had slipped briefly below $40 for the first time since February 2009.

Brent, the international benchmark, also sank sharply in London trade to its lowest level since early 2009.

"We're seeing equity markets in Asia implode and there definitely is a lot of risk-off sentiment," said Bart Melek at TD Securities.

The Shanghai stock market plunged 8.5 percent Monday, its sharpest one-day fall in eight years, as concerns mounted about the slowdown in China's economy, the world's largest energy importer.

The China rout rocked equity markets in Asia and Europe, and US stocks plunged in opening trade, pushing the Dow Jones Industrial Average down 5.8 percent.

Amid market turmoil over China's faltering economy, WTI shed 4.8 percent last week, marking its eighth consecutive weekly decline, the longest stretch of weekly losses in 29 years.