Venezuelan and Ecuadorean troops deployed on Colombia's frontier last night as South America's military and diplomatic crisis escalated into a dangerous showdown between President Hugo Chávez and Colombia's US-backed government.

Venezuela started shutting crossing points on the 1,400-mile border to try to isolate its neighbour after Bogotá made a series of extraordinary allegations about the Venezuelan leader funding Marxist guerrillas intent on building a uranium-enriched "dirty" bomb.

"Colombia proposes to denounce Hugo Chávez, president of Venezuela, in the international criminal court for sponsoring and financing genocide," said President Alvaro Uribe.

The Organisation of American States, a pan-regional body, held an emergency meeting in Washington to seek a diplomatic solution after President George Bush sided with Colombia, his administration's key ally in Latin America. The US president accused Venezuela of "provocative manoeuvres" and said he stood by Bogotá and its fight against terrorism. He also urged Congress to approve a free-trade agreement with Colombia.

Pictures of Venezuelan trucks and tanks rolling west, the vanguard of 10 battalions which were ordered to mobilise, and of Ecuadorean troops moving to the Colombian frontier from the other side of the Andes, underlined the risk of South America enduring its first war in over a decade. Quito and Caracas have severed diplomatic ties with their neighbour.

"A serious cross-border military conflict is unlikely [but] the chain of actions and reactions often has a life of its own. There is no telling where it could lead," said Michael Shifter, of the Inter-American Dialogue thinktank.

Colombia triggered the crisis last Saturday by bombing a rebel camp one mile inside Ecuador, killing at least 21 members of the Revolutionary Armed Forces of Colombia (Farc), including a senior commander, Raul Reyes.

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