A Mali separatist movement on Sunday said it was prepared to act against Islamic militants following the destruction of three sacred tombs in Timbuktu, and called on the United States and France "to help us kill them."

The tombs, a 15th-century shrine to Muslim saints, are part of a UNESCO World Heritage site in Timbuktu. UNESCO on Saturday condemned the destruction and called for it to stop.

The U.N. agency, which sponsors cultural, scientific and educational programs to further peaceful relations between nations, has received reports that three of the site's 16 mausoleums were "completely destroyed," UNESCO Director General Irina Bokova told CNN on Sunday. They are the mausoleums of Sidi Mahmoud, Sidi Moctar and Alpha Moya.

Timbuktu Mayor Ousmane Halle said that Muslims on their way to Friday worship at the tombs were stopped and threatened by armed men from Ansar Dine, a militant group that seeks to impose strict Sharia law.

Ansar Dine has threatened to continue the destruction.

"It's very, very bad for the Azawad people and the whole world what Ansar Dine has done in Timbuktu. We condemn it completely," Moussa Ag Assarid, spokesman for the Tuareg rebels' National Movement for the Liberation of Azawad, or MNLA, said Sunday. The group promotes a separate state it calls Azawad.

"We are ready and preparing to right with these terrorist organizations," Assarid said. "We will move soon, and (are) taking our arms and vehicles and material, but we still need help."

The MNLA claims Islamist extremist groups including Ansar Dine, Boko Haram and al Qaeda have seized control of Timbuktu along with the towns of Gao and Kidal, and have killed top MNLA leaders there.

"We call on the USA, France and all other countries who want to stand against Ansar Dine, Boko Haram and al Qaeda who are now holding Timbuktu, Gao and Kidal to help us kill them and help the people in those cities," Assarid said.

U.N. Secretary-General Ban Ki-moon is "concerned by the worsening security and humanitarian situation in northern Mali" and reports of the destruction, his office said in a statement. "Such attacks against cultural heritage sites are totally unjustified," the statement said.