MADRID - THE ARMED Basque separatist group ETA considered assassinating King Juan Carlos by attacking his plane or helicopter with a surface-to-air missile, Spain's interior minister said on Monday.

Alfredo Perez Rubalcaba told a news conference the plot was detected in 2004 when authorities seized ETA-related documents, but it never made it into the planning stages. He gave no further details.

The newspaper El Correo reported over the weekend that several CDs found in France at an ETA hideout in 2004 showed the group had considered attacking the king with a surface-to-air missile while he flew by plane or helicopter, firing on the aircraft from near an airport or military base.

The newspaper said French anti-terrorist police who found the CDs also found the remains of a spent missile cartridge, which it said showed ETA had carried out a real-fire test.

The CDs said Spain's prime minister or Cabinet members could also be targeted, the newspaper said.

'It is true that there was a theoretical study that went no further than that ... There was no concrete planning,' Rubalcaba said Monday.

In 1995, Spanish authorities thwarted a plot to shoot the king with a long-range rifle while he was on vacation in Palma on the Spanish island of Mallorca.

On Sunday, ETA issued a statement warning it would make the next Basque regional government - due to be led by non-nationalists for the first time - a 'priority target.'

The regional government had been led since 1980 by a party that flirted with independence for the region, but in March 1 elections of this year it failed to win a majority.

Now, the Basque branch of Spanish Prime Minister Jose Luis Rodriguez Zapatero's Socialist party will take charge with a minority government, relying on support from the conservative Popular Party. The government is due to be sworn in in the next few weeks. -- AP