US oil prices finished sharply lower Friday, weighed down by worries over excess supply and tepid demand amid global economic uncertainty.

The benchmark US contract, West Texas Intermediate for January delivery, dropped $4.21 to $50.42 a barrel in a holiday-shortened session, a decline of 7.7 percent and the lowest level in more than a year.

The Saudis have signalled they will throttle back production in December, but unless Opec and Russia can reach a new deal to constrain supply at their meeting next month, analysts see the prospect of sustained oversupply in 2019, undoing the group’s success over the last two years to drain global inventories.

Crude collapsed into a bear market this month after the US allowed some nations to continue buying Iranian crude. Trade tension between America and China is raising concerns over demand and President Donald Trump has renewed his call on Saudi Arabia to lower prices further. These factors pushed up oil’s volatility this week to the highest since 2016.

“Prices have taken a dive as Trump continues to put pressure on Opec and Saudi Arabia to create a low-price environment, and that has coupled with increasing American stockpiles,” said Hong Sungki, a Seoul-based commodities trader at NH Investment & Securities Co. “A potential game-changer will be what Opec+ agrees to do in terms of supply.”

Brent for January settlement fell as much as $2.89 to $59.71 a barrel on the London-based ICE Futures Europe exchange, and headed for a 11 per cent weekly loss. The global benchmark crude traded at a $8.70 premium to WTI. Total volume traded was 43 per cent above the 100-day average.

West Texas Intermediate for January delivery lost as much as $3.37 to $51.26 a barrel on the New York Mercantile Exchange. The contract is on course for a 8.9 per cent decline this week, the longest weekly stretch of drops since August. There was no settlement on Thursday due to US Thanksgiving holiday.

Earlier this month, the Organisation of Petroleum Exporting Countries and allied producers warned that markets will probably be oversupplied in 2019. US stockpiles expanded to the highest level since December 2017 last week and oil producers there are producing at the highest rate since at least March 1983, according to government data.

While indicating that Saudi Arabia is producing at record levels, Al Falih said the world’s biggest exporter won’t oversupply the market. Demand for Saudi crude may be lower in January compared with December, he told reporters on Thursday.