The US yesterday stepped up pressure on Venezuela, warning that its weapons procurement programme threatened to destabilise the region.

Comments by Donald Rumsfeld, US defence secretary, during a visit to Brazil were the strongest indicator yet of US concern about the planned acquisition of large quantities of Russian firearms by President Hugo Chávez's leftwing government.

"I can't imagine why Venezuela needs 100,000 AK-47s, I can't imagine what is going to happen to 100,000 AK-47s," Mr Rumsfeld said before a meeting with Luiz Inácio Lula da Silva, Brazil's president. "I personally hope the [delivery] doesn't happen . . . if it did, it wouldn't be good for the hemisphere."

US officials fear the imported weaponry could end up in the hands of the Revolutionary Armed Forces of Colombia (Farc), the leftwing guerrilla group. Some Chávez critics also fear the weapons could reach leftwing groups in Bolivia and elsewhere in Latin America. Tension over the issue is straining relations between the US and Venezuela.

But as Washington grows frustrated with Mr Chávez's anti-US rhetoric and suspected contacts with "destabilising forces" in the region, it is turning to Brazil as a moderator.

Mr Rumsfeld's trip to Brazil is part of a growing perception in Washington that the left-leaning Lula da Silva administration can be an ally in tackling regional instability. Previous US concerns, such as Brazil's uranium enrichment programme, appear to have been put to one side.

Mr Rumsfeld said: "Our two countries are looking at ways to work together more closely to confront the anti-social threats from organised crime, gangs, drug-traffickers, hostage-takers and terrorists."

He praised Brazil's peacekeeping efforts in Haiti as "a welcome contribution to stability in our hemisphere".

An aide accompanying Mr Rumsfeld on the tour added: "We wish Chávez would listen more to Lula."

Mr Lula da Silva, who maintains a cordial relationship with Mr Chávez, is to participate in a summit between the Colombian and Venezuelan heads of state and José Luiz Zapatero, Spain's prime minister, in Venezuela on Tuesday.