At least four people have been killed and 15 injured when suspected leftist rebels detonated a car bomb near a town where US Special Forces are due to train Colombian troops.
The bomb exploded on Thursday outside a military garrison in the village of Fortul, in the lawless, oil-rich eastern Arauca Province, some 250 miles (400 km) northeast of the capital of Bogota, near the border with Venezuela. All the victims were civilians, most of them shopkeepers and passers-by.
"The explosion caused the confirmed death of four people, but apparently there are five dead, and another 15 wounded. They are all civilians," said Fortul Mayor Fernando Trivino.
Government officials blamed the bomb attack - the second to rock the province in two days - on the 17,000-strong Revolutionary Armed Forces of Colombia, known in Spanish as "FARC."
On Wednesday, a suspected FARC rebel was killed when a car packed with 88 pounds (40 kg) of explosives accidentally blew up as he was driving through the village of Arauquita.
US Special Forces are scheduled to begin training Colombian troops this month to protect the Cano Limon pipeline, a favourite target of leftist rebels. The key pipeline serves an oil field operated by U.S. firm Occidental Petroleum.
The US personnel are to be stationed at military bases in nearby Arauca and Saravena, two violence-racked towns heavily infiltrated by rebels fighting in the four-decade civil war.
The United States has provided almost $2 billion of military aid to Colombia in recent years, mainly targeted at the country's massive cocaine trade.
Under state-of-emergency powers, President Alvaro Uribe last September declared Arauca, Arauquita and Saravena "special combat zones" in an effort to contain the violence.
But a constitutional court threw out many of the powers Uribe gave to the military in the special zones in November, saying they could not tap phones, make arrests or raid homes without warrants.
On Thursday, an unflappable Uribe reiterated his ultimatum to armed groups to negotiate peace or face military defeat.
"It should be clear that we have the will to seek peace with one group or another. If we achieve it, fine, if not we will defeat them. Our hand will not tremble. We have all the determination," Uribe told Caracol radio in an interview.
Uribe, a tough-talking 50-year-old lawyer who campaigned with the slogan "Firm Hand, Big Heart," took office in August promising to boost defence spending to stem violence that claims the lives of thousands of people every year.
But he has also sounded a more conciliatory note, saying he would open peace talks with any armed group on the condition they call a cease-fire.
Colombia's war pits leftist rebel groups against the U.S.-backed military and right-wing paramilitary outlaws who fight alongside the state security forces.
The main paramilitary force, the United Self-defence Forces of Colombia, known as AUC, declared a unilateral cease-fire on December 1. Uribe then named a group of officials to meet paramilitary commanders to "explore" possible negotiations.
In December, Colombia's second-largest Marxist rebel force, the Cuban-inspired National Liberation Army, or ELN, broke off preliminary talks with the government, accusing Uribe of preparing for war.
This week, Uribe invited Spain, France, Norway, Switzerland and Cuba to mediate in bringing the ELN back to the table. The FARC have refused Uribe's peace conditions.