The deadline for the lives of 23 South Korean aid workers being held in Afghanistan has been extended until Tuesday night, according to a purported Taliban spokesman.

Qari Yousef Ahmadi, who claims to speak for the Taliban, said late Monday the hardline militants extended the deadline another day after the Afghan government refused to release any of the 23 Taliban prisoners the insurgents want freed.

The extension to execute the hostages is the third by the captors. Afghan officials in Ghazni province have met the militants and are also negotiating over the phone, but little progress appears to have been made so far.

An Afghan lawmaker said the militants had upped their demands Monday, insisting all insurgent prisoners in Ghazni province be released in exchange for the Koreans. Ahmadi, however, denied the allegation.

Khail Mohammad Husseini, a lawmaker from Ghazni province, said a delegation of provincial leaders tried to meet with the kidnappers earlier Monday but the militants didn't show. He said the Taliban increased their demands by telephone, saying all militant prisoners in Ghazni had to be released.

Ahmadi said militants were still demanding the release of 23 Taliban prisoners but said the government hadn't signalled it was willing to do that.

"If the government won't accept these conditions, then it's difficult for the Taliban to provide security for these hostages, to provide health facilities and food," Ahmadi told The Associated Press by satellite phone. "The Taliban won't have any option but to kill the hostages."

The deputy interior minister, Abdul Khaliq, said Afghanistan was not prepared to make a deal "against our national interest and our constitution," though he did not explicitly rule out freeing any prisoners.

Afghan President Hamid Karzai in March authorized the release of five Taliban prisoners in exchange for a kidnapped Italian journalist, but he called the trade a one-time deal.

The move was criticized by the United States and European nations who felt the trade would encourage more kidnappings.

Ahmadi said Sunday the militants were giving the Afghan and South Korean governments until 7 p.m. (10:30 a.m. ET) Monday to respond to their demands.

Afghan elders leading the hostage negotiations met with the kidnappers on Sunday. They reported the Koreans were healthy.

The aid workers were kidnapped Thursday in Ghazni district while riding on a bus from Kabul to Kandahar on Afghanistan's major highway.

The Afghan military has the region surrounded in case the government decides the army should move in.

South Korea, meanwhile, banned its citizens from travelling to Afghanistan in the wake of the kidnappings, said Han Hye-jin, a Foreign Ministry official. He said Seoul also asked Kabul not to issue visas to South Koreans and block their entry into the country.

South Korea had previously asked its nationals to avoid visiting Afghanistan, citing the political instability.

The South Korean church that the abductees attend has said it will suspend at least some of its volunteer work in Afghanistan. It also stressed the captives, 18 of whom are women, were not involved in any Christian missionary work, saying they only provided medical and other volunteer aid to distressed people in the war-torn country.

South Korea has about 200 soldiers serving with the 8,000-strong U.S.-led coalition in Afghanistan, largely working on humanitarian projects. They are scheduled to leave Afghanistan at the end of this year.

With files from The Associated Press