Security forces in central Afghanistan launched an operation on Wednesday aimed at rescuing 21 South Korean hostages hours after a deadline set by their militant captors expired, a provincial official told Reuters.

"The operation has started," Reuters quoted Khowja Seddiqi, the district chief of Ghazni's Qarabagh district, as saying. Taliban militants are believed to have been holding the 16 women and five men in the area for almost two weeks.

Protesters shout slogans as riot police officers stand guard during a rally outside the U.S. Embassy in Seoul on Wednesday.
(Lee Jin-man/Associated Press)

The report came after Afghanistan said for the first time it will not release insurgent prisoners in any potential exchange for the Christian aid workers, which has been the militants' key demand throughout negotiations.

A purported Taliban spokesman had set a deadline of noon (3:30 a.m. ET) Wednesday for their fighters to be released and threatened to kill the remaining hostages.

Seddiqi said the hostages were still alive after the deadline passed.

Afghan forces have converged on Ghazni for about a week, but have held back from advancing on the area where they believe the hostages are located while talks with the militants continued.

Before the operation reportedly commenced, the army had dropped leaflets that warned it would launch an operation to rescue the hostages.

"The Defence Ministry wants to launch a military operation in the area," the Associated Press reported the leaflets as saying. "In order for you to be safe and not be affected by the operation, we call on you to move to secure government-controlled areas."

Some hostages 'seriously ill': Taliban spokesman

Before Wednesday's deadline, Qari Yousef Ahmadi, who claims to speak for the Taliban, said two women being held were seriously ill with unknown medical conditions and the captors had no medicine to help them.

"The majority of the hostages are ill, but two females are seriously ill and there is this possibility that they may die," Ahmadi told Reuters by telephone from an undisclosed location.

The South Koreans were kidnapped while riding a bus July 19 on the Kabul-Kandahar highway. They are the largest group of foreign hostages taken in Afghanistan since the 2001 U.S.-led invasion that drove the Taliban from power.

The militants have extended several previous deadlines without consequences, but killed 29-year-old Shim Sung-min on Monday after a deadline passed. His body, with a gunshot wound to the head, was found along a road in Andar district. 

His death came a week after kidnappers shot and killed hostage Bae Hyung-kyu, a 42-year-old pastor who was the group's leader.

Afghan President Hamid Karzai faced criticism when he approved the release of five Taliban prisoners earlier this year to free an Italian journalist held hostage. He subsequently vowed not to repeat such a deal.

With files from the Associated Press