Flows of products used to make premium gasoline from Asia to the US are at the highest level in three months as American fuel producers grapple with tightening supplies during a period of peak demand.

Shipments of products such as alkylate and reformate are at around 86,000 barrels a day so far in June, according to preliminary data from analytics firm Kpler. Known as blendstocks, they can be mixed with gasoline to boost the octane rating, or quality of the fuel. A lack of refining capacity across many regions including the US has crimped output of the components.

US gasoline prices have repeatedly broken records and the national average recently topped $5 a gallon as the summer travel period ramps up. The White House is considering limiting exports of fuels as President Joe Biden intensified his criticism of soaring profits for oil companies. Overall gasoline stockpiles are at the lowest seasonal level in seven years.

Efforts to build national stockpiles are being hampered by strong fuel demand from Latin America. Waterborne gasoline and diesel loadings out of the US Gulf Coast so far in June put monthly volumes on track for a record in data going back to January 2016, according to oil analytics firm Vortexa.

Better quality or high-octane gasoline recently rallied to the widest premium ever over regular fuel in New York as a shortage boosted prices for the grade that accounted for almost 13% of total US sales last year. On the West Coast, California is facing even higher prices as refineries shut units for maintenance.

“The pull for blendstocks to produce more gasoline is not surprising” given high motor-fuel prices in the US, said Jane Xie, a senior oil analyst at Kpler. The majority of shipments from Asia are heading to the Atlantic Coast, according to Kpler data. Motor fuel stockpiles in the region have dwindled.

Market dynamics are also putting additional demand pressure on blendstocks. Excess naphtha is being mixed with gasoline because it’s trading at wide discount to the motor fuel. However, because naphtha is octane-poor, often blending components are needed to boost the octane rating of gasoline.