By Jeremy McDermott BBC correspondent on Medellin

The cache was uncovered in a region where the guerrillas have been destroying the infrastructure.

The rebels have caused widespread disruption attacking power lines

But in a setback for the government, the United Nation's representative to Colombia said that anti-terrorist legislation which is making its way through Congress should not be passed as it violates basic human rights.

The discovery of five tonnes of high explosives in a rubbish dump in the southern province of Narino marks the largest seizure ever in Colombia.

The authorities are hoping that the capture of this FARC explosives store will bring an end to the wave of infrastructure bombings the guerrillas have conducted in the south-western part of the country.

More than 25 electricity pylons and many communications towers have been blown up during May, leaving parts of the region without power or phone lines.

Privacy under threat

But it is not all going the government's way. Michael Fruhling, the UN representative to Colombia, gave a press conference condemning some parts of the anti-terrorist legislation the government is trying to get through Congress.

He said that many elements of the bill should not be passed as they invade citizens' privacy and would constitute an abuse of basic human rights.

The government insists that it needs the powers granted under proposed legislation to prosecute the war on terrorism.

But the Colombian security forces do not have a good human rights record and some NGOs fear that sweeping new powers will lead to yet more abuses.