An unknown militant group holding an American working for the United Nations in Pakistan has said it will kill the hostage, John Solecki, in four days if its demands are not met.

The Baluchistan Liberation United Front (BLUF) gave the latest deadline in a letter delivered to a local news agency in Quetta late on Sunday.

Solecki, 49, the head of the office of the U.N. High Commissioner for Refugees (UNHCR) in southwestern Baluchistan province, was kidnapped in the provincial capital of Quetta on Feb. 2 after gunmen ambushed his car and shot dead the driver.

"If our demands are not met then we will kill him, and state agencies will be responsible for it," the group said in the letter.

The group attached a list of 1,109 names of people it said were missing and being held by Pakistani security agencies, and demanded their release.

This is not the first deadline Solecki's captors have given.

The BLUF had earlier demanded U.N. intervention to secure the release of 141 women it said were held in Pakistani torture cells, provide information about more than 6,000 missing persons, and resolve the issue of Baluch independence under the Geneva Convention.

Baluchistan, the largest but poorest of Pakistan's four provinces, lies on the border with Afghanistan. Separatist militants have fought a low-scale insurgency there for decades.