A pump jack and pipes at an oil field near Bakersfield, California.

Oil prices slumped on Monday, extending steep declines from Friday, as Saudi Arabia and Russia said they may increase supplies and as U.S. production gains show no signs of abating.

Brent crude futures were at $75.09 per barrel at 0452 GMT, down $1.35, or 1.8 percent, from their last close.

U.S. West Texas Intermediate (WTI) crude futures were at $66.22 a barrel, down $1.66, or 2.5 percent.

Brent and WTI have fallen by 6.4 percent and 9.1 percent respectively from peaks touched earlier in May.

In China, Shanghai crude oil futures tumbled by 4.8 percent to 457.7 yuan ($71.64) per barrel.

The Organization of the Petroleum Exporting Countries (OPEC), as well as top producer but non-OPEC member Russia, started withholding supplies in 2017 to tighten the market and prop up prices, which in 2016 fell to their lowest in more than a decade at less than $30 per barrel.

But prices have soared since the start of the cuts last year, with Brent breaking through $80 per barrel earlier in May, triggering concerns that high prices would crimp economic growth and stoke inflation.

"The pace of the recent rise in oil prices has sparked a debate among investors on whether this poses downside risks to global growth," Chetan Ahya, chief economist at U.S. bank Morgan Stanley, wrote over the weekend in a note.