'It is essential to grant some kind of formal guarantee, a legal guarantee, that possible (US military) operations will happen strictly within Colombian territory,' Brazilian presidential spokesman Marcelo Baumbach stressed Friday.

'There is a suggestion for there to be greater transparency,' Brazilian Foreign Minister Celso Amorim had said earlier.

According to Amorim, Lula hopes all doubts can be dispelled 'calmly, based on technical considerations' at the Unasur summit. Lula stressed that he favours 'bilateral and multilateral cooperation between South American nations to combat drug trafficking, without external interferences'.

The Unasur summit is scheduled to coincide with the inauguration of Rafael Correa Monday for a further term as Ecuadorian president. Correa, who took office in January 2007, moved to change the country's constitution and was re-elected under new rules in April.

Ecuador has invited ousted Honduran President Manuel Zelaya to the inauguration ceremony. Indeed, Unasur is certain to issue a formal condemnation of the coup that removed Zelaya from power June 28.

Zelaya, a democratically-elected leader, had tried to change the Honduran constitution so he, too, could run for re-election, triggering opposition forces to send him into exile.

At this summit, Ecuador is set to take over the rotating presidency of Unasur, an organisation that was formally set up in May 2008 and brings together the region's 12 countries.