Story highlights U.N. official: 26 aid workers have been killed in Afghanistan this year

A Czech humanitarian group says those killed overnight were its staffers

They worked with National Solidarity Program, set up to help with development projects

Kabul, Afghanistan (CNN) Militants killed nine aid workers in an overnight attack on a house where they were staying in Afghanistan's northern Balkh province, a local official said Tuesday.

The nine were working with the Afghan National Solidarity Program, according to Abdul Razaq Qaderi, chief of the criminal department for Balkh province.

A Czech humanitarian organization, People in Need, said it employed the workers, who were Afghan nationals. They were attacked at a field office in the Zari district of Balkh, it said.

The investigation is ongoing, People in Need said.

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