(Bloomberg) -- US gasoline futures have hit a six-month high as disruptions at Russian oil refineries and dwindling US fuel stockpiles exacerbate supply concerns ahead of summer driving months.

A flurry of Ukrainian drone attacks on Russian oil refineries in the past week have increased the geopolitical risk premium and left markets on edge. Wholesale fuel distributor TACenergy estimates that the outages are around 24% of Russia’s estimated refining capability — or just over 1% of global capacity. That means even more tightness in markets after US gasoline stockpiles dropped to the lowest level since November.

Futures in New York rose Monday to the highest intraday level since September, touching $2.77 a gallon. Such a gain can lift crude prices, since tighter fuel supplies tend to drive up demand for oil. The gasoline crack spread — a measure of profitability of turning crude into the fuel — climbed to the highest since August, emphasizing gasoline’s strength.

Read More: Global Fuel Prices Are Surging With Supply Risks Ahead

Gasoline prices also typically rise heading into April as refiners make the switch from winter-grade fuel to the more expensive summer grade. Prices at the pump are around $3.50 a barrel, the highest seasonal average in two years, according to the AAA automobile club.

Crude rallied Monday to a four-month high, which could mean added pressure for North American drivers ahead of the busiest months for road trips.

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