U.S. crude was back in a bull market Tuesday as Turkey threatened to shut down Kurdish oil shipments while Trafigura Group and Citigroup Inc. added to warnings of a looming supply squeeze.

Futures were little changed in New York after closing more than 20 percent above their most recent low on Monday, a definition of a bull market.

Turkey can “close the valves” on oil exports from Kurdistan, Turkish President Recep Tayyip Erdogan said as the region in Iraq’s north voted in a referendum on independence. There’s a risk of a market squeeze emerging as early as 2018 because of weaker investment in exploration and development, Citigroup’s head of commodities research said.

Oil has gained more than 10 percent this month on forecasts for rising crude demand and as members of the Organization of Petroleum Exporting Countries continue to trim output to drain a global glut. The landlocked Kurdish enclave can ship as much as 700,000 barrels a day through the pipeline to the Turkish port of Ceyhan on the Mediterranean.

“That quantity of crude coming out of the supply chain would be fairly significant,” said David Lennox, an analyst at Fat Prophets in Sydney. “The price reaction might indicate that the supply situation is a little closer to balance. These types of geopolitical events tend to drag out, it could certainly help to keep prices higher for longer.”

West Texas Intermediate for November delivery was at $52.18 a barrel on the New York Mercantile Exchange, down 4 cents at 2:55 p.m. in Hong Kong. Prices surged $1.56, or 3.1 percent, to $52.22 on Monday to the highest close since April 18. Total volume traded was 55 percent above the 100-day average.

Brent for November settlement was 11 cents higher at $59.13 a barrel on the London-based ICE Futures Europe exchange after rising as much as 0.8 percent earlier. Prices added $2.16 to $59.02 on Monday, the highest close since July 2015. The global benchmark traded at a premium of $6.92 to WTI.

Oil pumped at fields controlled by the Kurdish Regional Government and the central Iraqi government’s North Oil Co. was flowing normally through the export pipeline on Monday, according to two people familiar with the matter, who asked not to be identified because the information is confidential.

The crude market could face a shortage by 2019, Trafigura Group’s Co-Head of Group Market Risk Ben Luckock said at the at S&P Global Platts APPEC conference on Tuesday. Nine million barrels a day of oil production could be lost to well declines by 2019, according to the trader.

Oil-market news:

• The OPEC-led group needs to prolong cuts to reduce inventories to historically normal levels, according to Janet Kong, Eastern Hemisphere chief executive officer of integrated supply and trading at BP.