Venezuela was today completing a big military buildup on its border with Colombia as the president of Ecuador, the other player in the region's most perilous crisis for years, said his country would risk the "ultimate consequences" to defend itself.

While analysts still believe warfare between Colombia and its neighbours to the north and south is unlikely, with thousands of heavily armed troops arriving in border regions the situation remains volatile and unpredictable.

A separate, diplomatic front has turned into a test of strength between the leftwing presidents of Ecuador and Venezuela and the US-backed administration in Bogotá.

Enraged by a Colombian bombing raid on Saturday on a rebel camp one mile inside Ecuadorean territory, Ecuador has already sent 3,200 troops to its northern border with Colombia.

A top Venezuelan general today said 90% of a force of around 9,000 troops had arrived in regions on the Colombian border, following a weekend mobilisation order by the country's president, Hugo Chávez, which also saw the deployment of tanks and aircraft.

The attack on Saturday, which killed at least 21 members of the Revolutionary Armed Forces of Colombia (Farc), including a senior commander, Raúl Reyes, also prompted condemnation from Brazil, Chile, Peru, Mexico and Argentina.

The president of Ecuador, Rafael Correa - who has rejected one Colombian apology as insufficient - was in Brazil today for the latest leg of a rapid six-nation regional tour intended to whip up support against Colombia.

Speaking after a meeting with Brazil's left-leaning president, Luiz Inacio Lula da Silva, Correa demanded that the Organisation of American States, which is meeting in Washington for a second day today to discuss the crisis, "take a stand quickly".

"Otherwise we will have to defend ourselves by our own means," Correa told reporters. "I insist on this: Ecuador is ready to go to the ultimate consequences."

Colombia's president, Álvaro Uribe, receiving strong support from George Bush, has hit back with allegations of collusion between Farc – designated a terrorist group by the US and EU – and the Ecuadorean and Venezuelan leaders.

Despite the martial rhetoric, the likelihood of war is greatly reduced by the three countries' heavy dependence on mutual trade, especially the transit of food into Venezuela through Colombia. Additionally, Venezuela's armed forces are widely considered no match for the US-equipped Colombian military.

Colombia has so far opted not to deploy any extra forces on its borders, relying instead on a concerted diplomatic offensive based around what it says were extraordinary discoveries gleaned from files on Reyes's laptop, seized in the raid.

The Colombian government said Chávez received money from the drug-funded guerrillas in 1992, when he was an impoverished failed coup-monger with political ambitions, and that he recently gave the rebels $300m (£150m).

If the latter allegation is substantiated Chávez could in theory be prosecuted, since internationally Farc is categorised as a terrorist organisation. Yesterday, Uribe called for Chávez to be tried by the international criminal court. A Venezuelan government minister said the allegation was a smear.

At a UN disarmament meeting in Geneva, Colombia's vice-president, Francisco Santos, made a further extraordinary claim, saying the seized files revealed the guerrillas were negotiating to obtain radioactive material and hoped to make a "dirty bomb".

Documents Colombian officials released to reporters did not support this allegation, indicating instead Farc only discussed the possibility of buying uranium to resell at a profit.

Farc said yesterday Colombia's raid had gravely damaged the chances of releasing more of the 700 hostages it holds in jungle camps, including Íngrid Betancourt, the ailing Franco-Colombian politician who has become the public face of the captives' plight.

The rebels said Reyes died completing a mission to arrange Betancourt's release through Chávez and the French president, Nicolas Sarkozy, who has made it a personal cause. Sarkozy said last week Betancourt could be near death, and that her "martyrdom [would be] the martyrdom of France".