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CALI, Colombia (CNN) -- Colombia's largest and most feared right-wing paramilitary group freed two human rights workers Thursday, three weeks after they were kidnapped at gunpoint from their headquarters in Medellin.

The two men were among four directors of the Popular Training Institute rights group seized January 28 by the United Self-Defense Forces.

Two women were released February 8. But the paramilitary group had vowed it would hold onto the institute's executive director and human rights director as "prisoners of war," accusing them of ties to leftist Colombian rebels.

The Colombian government's representative for human rights, Jose Fernando Castro Caicedo, said the men were released Thursday at an undisclosed location in northern Colombia.

Paramilitary groups have often targeted human rights organizations, claiming they act as fronts for rebel sympathizers. The international group Human Rights Watch says Colombia is one of the most dangerous countries for activists. It says more than 20 rights workers have been killed in the last two years.

Sierra pledges to continue working despite death threats

"Each morning, we go to work a different way. You have to change your routine, change your activities," human rights worker Jose Efrain Sierra said. "Some of our workers have been kidnapped or killed in traffic."

But despite the dangers, Sierra pledged to continue working under the shadow of death threats.

"This is the reason for being for many of us," he said. "If this is what you believe in, you have to do this work and accept the consequences."

Sierra and most other rights workers seek an end to fighting among rebels, the government and paramilitary groups. Nearly 30,000 people died last year in the civil war and related street violence. More than 1,200 civilians were massacred, primarily by paramilitaries, Castro said.

Sierra and other activists want the government to pay more attention to human rights. They say Colombia has begun taking the problem more seriously, especially at the local level.

The city of Cali operates a 24-hour hotline taking reports of abuses.

"We have made progress," said public advocate Noralba Garcia. "The state is starting to investigate more seriously to try to end the impunity of the killings."

For Sierra, that's a small start, but a reason for optimism that one day, peace will come to Colombia.

Correspondent Frida Ghitis and Reuters contributed to this report.

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