Car users in the North Korean capital were scrambling Friday to fill up their tanks as gas stations began limiting service or even closing amid concerns of a spreading shortage.

A sign outside one station said sales were being restricted to diplomats or vehicles used by international organizations, while others were closed or turning away local residents. Lines at other stations were much longer than usual and prices appeared to be rising significantly.

The cause of the restrictions or how long they might last were not immediately known. Supply is controlled by the state.

North Korea relies heavily on China for its fuel supply, and Beijing has reportedly been tightening its enforcement of international sanctions aimed at getting Pyongyang to abandon its development of nuclear weapons and long-range missiles.

The issue was raised at a regular Chinese Foreign Ministry news conference in Beijing on Friday after a Chinese media outlet, Global Times, reported that gas stations were restricting service and charging higher prices.

But spokesman Lu Kang gave an ambiguous response when asked whether China was restricting fuel deliveries.

“As for what kind of policy China is taking, I think you should listen to the authoritative remarks or statements of the Chinese government,” he said, without elaborating on what those remarks or statements are. “For the remarks made by certain people or circulated online, it is up to you if you want to take them as references.”

One of China’s top North Korea scholars, Kim Dong-jil, director of the Center for Korean Peninsula Studies of Peking University, said he had not heard of new restrictions on fuel to pressure Pyongyang, but he said they are considered to be an option.

China’s Ministry of Commerce had no immediate comment.

Gasoline was selling at $1.25 per kilogram at one station, up from the previous 70 to 80 cents. According to a sign outside a station where ordinary North Korean vehicles were being turned away, the restrictions took effect Wednesday.

Gasoline is sold in North Korea by the kilogram, roughly equivalent to a liter (0.26 gallon).

When buying gas in North Korea, customers usually first purchase coupons at a cashier’s booth for the amount of fuel they want. After filling up the tank, leftover coupons can be used on later visits until their expiration date. A common amount for the coupons is 15 kilograms (19.65 liters or 5.2 U.S. gallons).

The military, state ministries and priority projects have the best access. Several chains of gas stations are operated under different state-run enterprises — for example, Air Koryo, the national flagship airline, operates gas stations, as well.