Bolivia's powerful farmers' union has agreed to join a week-old blockade of major roads to support coca growers, who are demanding an end to a government eradication campaign.

If the blockade is successful it will cut off the roads to the capital La Paz and other major cities around the country.

If the government maintains its intransigent position, we'll paralyse this nation until the government is forced from office Evo Morales, coca growers' leader

The coca farmers want an end to the US-funded eradication programme.

The government says illegal coca crops are used to produce cocaine, but the farmers say the coca is intended for traditional uses.

At least six farmers have died and 50 others have been injured in violent clashes with police trying to clear the blockade.

On Tuesday a police officer was killed and three others were wounded in an ambush.

Stand-off hardens

The blockade by coca growers, or "cocaleros", has been high-profile but so far relatively ineffectual.

Now, Evo Morales, the leader of the nation's coca growers, and Felipe Quispe, the leader of Bolivia's largest workers union, have joined forces.

"We're open to dialogue, but if the government maintains its intransigent position, we will paralyse this nation until the government is forced from office," Mr Morales said.

Mr Morales, an indigenous member of congress, only narrowly lost last year's presidential election on his promise of support for coca growers.

If the blockade works, no fresh produce or fuel will be able to get in or out of La Paz and there will be no trade along the main routes to Chile or Peru.

Earlier this week, a new guerrilla group called the Army of National Dignity announced that it too will fight against the coca eradication campaign.

The government has dismissed the guerrillas as nothing more than a handful of disgruntled farmers, but correspondents say the dispute seems to be growing, not fading away.

The government has said it wants to negotiate, but it has also threatened to bring out the military to reopen the roads.