Spain's pilots' union opposed Wednesday the use of the military to replace air traffic controllers on sick leave, after the government proposed the move to undermine what it suspects is an undercover strike.

It would be difficult to maintain "the same capacity and the same levels of security" if the measure is adopted because military and commercial air traffic controllers follow "completely different" rules, Copac representative Gustavo Barba told public radio RNE.

Transport Minister Jose Blanco said Tuesday that the government would "in exceptional situations" employ military air traffic controllers to ensure flights go ahead.

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 will be held, causing delays to dozens of flights, he added.

The situation appeared to have returned to normal on Wednesday with 13 out of the 61 air traffic controllers who were scheduled to work at Barcelona's airport away on sick leave, a spokeswoman for airport management company AENA told AFP.

The only delayed or cancelled flights have been due to a strike by air traffic controllers in neighbouring France, she added.

The union representing Spain's air traffic controllers, USCA, on Wednesday again denied that an undercover strike was underway and said using military air traffic controllers would be "imprudent" and "dangerous".

Training would be given to military air controllers before the end of the month, Secretary of State for Transport Concepcion Gutierrez said Tuesday.

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, Valencia and the Balearic Islands that include tourist hot-spots Majorca and Ibiza.