Oil extended a surge from the lowest level in almost six years on speculation some investors are buying contracts to close out bearish bets. Gasoline rose as a refinery strike entered a second day.

West Texas Intermediate futures gained as much as 4.8% while Brent increased as much as 5% before paring those advances. Hedge funds and other speculators held the largest number of short contracts in WTI in four years. Oil rallied 8.3% on Friday as drillers pulled 94 rigs from US fields last week, the most on record. Gasoline jumped to a five-week high.

“Traders are stepping in and buying the market again,” said Gene McGillian, a senior analyst at Tradition Energy in Stamford, Connecticut. “This is a continuation of some of the short-covering that we saw on Friday. The market is watching for signs of slowing oil production.”

Oil prices fell to the lowest level since 2009 last month as the US pumped the most in three decades and OPEC kept its own supplies unchanged to defend its share of the global market. The strike by oil workers at plants accounting for 10% of US refining capacity continued Monday in the biggest walkout since 1980.

WTI for March delivery rose 89 cents, or 1.8%, to $49.13/bbl at 9:30 a.m. on the New York Mercantile Exchange after reaching $50.56. The volume of all futures was more than double the 100-day average.

Brent for March settlement increased $1.06, or 2%, to $54.05/bbl on the London-based ICE Futures Europeexchange after rising to $55.62. The European benchmark crude traded at a premium of $4.75 to WTI.

“The market was oversold before and the sentiment was extremely negative so on a way up the shorts are being grilled, which is in itself causing prices to rise further,” said Eugen Weinberg, head of commodities research at Commerzbank AG in Frankfurt.

Gasoline for March delivery advanced 2.6% to $1.517/gal on the Nymex after reaching $1.568, the highest since December.

The United Steelworkers union that represents employees at more than 200 refineries, terminals, pipelines and chemical plants stopped work Sunday at nine sites after failing to agree on a renewed labor contract. The USW hasn’t called a strike nationally since 1980, when a stoppage lasted three months.

The refineries on strike can produce about 1.82 million bpd of fuel, data compiled by Bloomberg show. They span the US, from Tesoro’s plants in Martinez, California; Carson, California; and Anacortes, Washington, to Marathon Petroleum’s Catlettsburg complex in Kentucky and three sites in Texas, according to the USW’s statement.

The US oil rig count dropped to a three-year low of 1,223 last week, Baker Hughes said Jan. 30. Drillers idled 352 oil rigs in eight weeks.

Rising US supply contributed to a global surplus that drove oil prices almost 50% lower last year. The Organization of Petroleum Exporting Countries, which has resisted calls to cut output, boosted production in January as Iraq pumped at a record pace, according to a Bloomberg survey of oil companies, producers and analysts.

Hedge funds and other big money managers cut their net-long positions in WTI 379 contracts to 216,325 futures and options in the week ended Jan. 27, according to the Commodity Futures Trading Commission. Short bets jumped 11% to 104,763, the most since 2010.