Brent crude rose to a three-year high of $70 a barrel last week, but there are questions about the rally’s resilience as higher oil prices threaten to unleash a new wave of U.S. shale output.

“The higher the prices go, the harder shale producers pump,” Tamas Varga at London-based broker PVM, said. He added, an “overdue correction could be under way.”

Oil prices have been rising since late June, supported by supply cuts led by OPEC producers and allies outside of the cartel such as Russia, which helped to curb excess stockpiles.

Brent, the international oil benchmark, on Jan.11 hit the highest level since December 2014, spurring concern that higher prices will attract increased US shale production, undermining efforts by global producers to cut supplies and triggering renewed pressure on prices.

Bijan Zanganeh, Iran’s oil minister, said last week that some Opec members were not keen on increased prices, especially of more than $60, while the U.S. Department of Energy forecast record levels of production this year, with output escalating to 11 million barrels a day (MMbbl/d) in 2019 up almost 20% from last year.

Still, UAE energy minister, Suhail Al Mazrouei, told an energy conference on Jan. 12: “I don’t think any fundamentals have changed for us to consider [a change in the output deal] or panic.”

Demand has remained robust in the winter months, while strong economic growth—which drives oil consumption—and a weaker U.S. dollar have also helped to buoy prices too. In the U.S., commercial crude inventories in the week to Jan. 5 fell by almost 5 MMbbl to 419.5 MMbbl, the lowest since March 2015.

Cyril Widdershoven, who runs the Verocy consultancy said: “This price level is sustainable for 2018 as demand grows while supply remains under pressure. OPEC and Russia are still committed to the cuts deal and any resurgence from US shale fields is still not enough.”

Also propelling prices have been geopolitical tensions in big oil-rich countries. The possibility of snapback sanctions on Iran’s energy sector and skirmishes between the Saudi-led coalition fighting in Yemen and Houthi fighters have drawn traders’ attention.