Spain said Tuesday it could deploy the military to replace Barcelona air traffic controllers on sick leave, in a bid to curb flight delays and undermine what it suspects is an undercover strike.

The level of staff on sick leave in Barcelona's control centre had fallen slightly to 32 percent on Tuesday from 36 percent on Monday, Transport Minister Jose Blanco said to Cadena Ser radio.

"In exceptional situations we will employ aerial military controllers to guarantee aviation traffic in our country," the minister said. "We are obliged to take alternative measures."

Training would be given to military air controllers before the end of the month, Secretary of State for Transport Concepcion Gutierrez said in an interview with public radio station RNE.

Blanco said there would be an investigation into the sick leave taken in the past few days by almost half the air traffic control staff at the control centre in Barcelona.

He raised the possibility of an undercover strike, saying he suspected the absences followed "a strategy" and could be "an infringement of law."

The situation seemed to have improved Tuesday morning, Spain's airport and air traffic authority Aena told AFP. "There are no more absent controllers than usual, it is a normal day for the moment," said a spokesman.

The number of days taken off by staff due to illness, union-related activities and for unjustified reasons have tripled in the past few months, according to Aena.

The Air Traffic Controllers Union reiterated that it was not behind the high rate of absences. "We are not on strike," the head of the union Camilo Cesa said at a press conference.

Cesa said the situation in Spain's airports was completely normal and staff sick leave was mostly due to stress brought on by working conditions.

Replacing air traffic controllers with the military would be "rash" and "dangerous", he said.

"We think this decision is rushed because (the military) do not have the same training as us, and there are clear differences between their work and ours," the union leader said.

Mariano Casado, who heads the union representing Spanish military air controllers, said the military "had the ability" to handle civil aviation but would need a time to adapt.

In February the Spanish government put an end to what it described as the "incomprehensible privileges" of air traffic controllers, including their high rates of pay and benefits.

At the beginning of this month, the air traffic controllers union and Aena resumed negotiations over a collective agreement.

The staff shortages have affected the airports in Catalonia, the Balearic Islands that include tourist hot-spots Majorca and Ibiza, and the Valencia region.

Spain is not the only European country experiencing air traffic control problems.

In France, a strike by controllers Wednesday against integration into a European system is set to cause widespread disruption, including the cancellation of one in five flights from the main Paris hub Roissy and half of all flights from smaller Orly airport.

Last week Greek air traffic controllers shut airspace for four hours in a dispute over pension reforms.