Portugal's Independent Union of Freight Drivers (SIMM) and National Union of Hazardous Drivers (SNMMP) announced on Saturday their decisions to continue the planned indefinite strike starting on Monday.

"The strike is to go on indefinitely," SIMM President Jorge Cordeiro told reporters after a joint meeting of the two unions in Lisbon on Saturday afternoon.

Portugal is declared by the government in a situation of an energy crisis from Saturday until the mid-night of Aug. 21 due to the strike of the fuel-tanker drivers.

Earlier in the day, Prime Minister Antonio Costa vowed to exercise its "own powers" by taking measures if the driver strike goes ahead and the minimum services set are not fulfilled.

"We will take the necessary measures," he said after an emergency meeting of ministers.

"We have done everything to prevent the strike from happening," he said, adding that his government will only move on to subsequent steps "if, when and to the extent strictly necessary".

The government has ordered minimum services of between 50 percent and 100 percent during the strike to ensure the security and public health services, after the unions and employers' association failed to reach an agreement over the drivers' pay rise.