The government on Saturday rejected a proposal from leftist rebels to swap a dying boy's policeman father for an imprisoned guerrilla.

The offer by the Revolutionary Armed Forces of Colombia to exchange Cpl. Norberto Perez for a rebel said to be ailing in jail was seen by many Colombians as another outrage in a tale that has angered a country weary of the rebels' abuses.

Twelve-year-old Andres Felipe Perez has cancer and has asked guerrilla leaders to release his father so they can see each other before it's too late. Doctors say the boy could die within days.

The government said Saturday that the rebels, known as the FARC, should release the father without condition.

Human rights activists supported the decision.

"By demanding payback for a simple humanitarian gesture, they're extorting an entire country," Eduardo Cifuentes, Colombia's top human rights official, told The Associated Press on Saturday.

On Friday, an Internet site run by FARC sympathizers posted a letter from rebel commanders saying they would free Perez if the government released Ignacio Gonzales Perdomo, a jailed comrade they described as "sadly ill."

The FARC, the largest of Colombia's two rebel armies, has held Perez nearly two years. Even though hostages are often permitted to watch television or listen to the radio, it is unknown if Perez knows of his son's condition.

The story of Andres Felipe, who is bald and gaunt from chemotherapy, has gripped the nation since the family announced a few weeks ago that his father was the only one who could save his life by donating a kidney.

FARC commanders offered to release the policeman but only if the boy underwent examination by rebel doctors within a safe haven that the government granted the FARC in 1998 to get peace talks going. The boy's doctors said he was too sick to travel.

The plight of Andres Felipe has generated anger across Colombia, where thousands of people are killed or kidnapped annually in the 37-year-old civil war pitting the guerrillas against government troops and a right-wing paramilitary group.

In more bloodshed, the army said suspected rightist militiamen executed at least 15 passengers forced off a bus Saturday near Sogamosa in Boyaca province, some 110 miles northeast of Bogota.

The gunmen, believed to be members of the United Self-Defense Forces of Colombia, singled out their victims and shot them, said army Gen. Freddy Padilla. He said as many as 17 people might have been killed.

The slayings came just two days after another massacre blamed on the right-wing group. Authorities said rightist gunmen took 12 people from their homes in Antioquia province and killed them Wednesday.