LISBON (Reuters) - Fuel-tanker drivers raised the specter of empty filling stations and aviation fuel shortages during Portugal’s busy tourist season on Tuesday, saying they would comply with a government order to go back to work but would not do extra time.

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Drivers began an indefinite strike on Monday, the second of the year after similar damaging industrial action in April, demanding better salaries and working conditions.

The government ordered drivers back to work late on Monday after supplies ran down at some locations including Lisbon airport.

Pedro Pardal Henriques, vice president of National Hazardous Materials Drivers’ Union, said drivers would work normal eight-hour shifts as required by the order but would do no extra time.

“This means they will do about half their normal hours,” Pardal Henriques said on Tuesday morning, adding that “gradually the filling stations will run empty”.

Early on Tuesday, nearly a third of Portugal’s filling stations remained either completely out of fuel or were partially dry, according to a website monitoring the strike.

The tourist-packed Algarve in the south has been hard hit by the strike.

Authorities said only 25 out of a scheduled 119 cargoes of fuel arrived at Lisbon airport during Monday, although they did not specify how many of those were for aviation fuel.

Airport authority ANA has said limited aviation fuel supplies have curbed operations but no flight cancellations have been reported so far.

EasyjetEZJ.L airline advised passengers traveling to and from Portugal to check the status of their flight due to the drivers' strike.

The government declared an energy crisis on Friday ahead of the strike, in an attempt to provide full supplies to ports, hospitals, airports and other priority locations. Measures adopted included rationing at petrol stations.

The special measure on Monday ordering drivers back to work means they could ultimately face imprisonment or lose their jobs if they do not follow the order. It also allows the government to mobilize the army to drive and escort petrol trucks, if necessary.

“There may be some who think there are no consequences of the civil requisition, but there are and there have to be,” Labour Minister Jose Antonio Vieira da Silva told RTP radio on Tuesday.