Afghan militants holding three United Nations relief workers hostage are in negotiations with groups trying to win their release.

A spokesman for the militant group "Army of Muslims" said Tuesday the talks are taking place, but he did not mention if any progress had been made.

Afghan officials say they are optimistic the U.N. workers will be freed unharmed.

The militants threatened to kill the hostages Wednesday if their demands for the withdrawal of U.N. and all foreign troops from Afghanistan, as well as the release of all Afghans held in foreign prisons, are not met. On Sunday, they offered to consider extending the deadline.

The hostages from Northern Ireland, Serbia and the Philippines were kidnapped from the Afghan capital, Kabul, last Thursday.

The U.N. relief workers were in Afghanistan to oversee the country's first direct presidential election on October 9.

Some information for this report provided by AFP and AP.