Pakistani military officials say friendly fire killed 13 paramilitary troops in the country's northwest as they were battling a militant ambush.

The officials said the attack took place late Monday in the Khyber tribal region along the Afghan border, and that the dead included a colonel and a captain in the Frontier Corps.

A top military commander said Tuesday the troops were killed by their own mortar.

Khyber lies on the main NATO land supply route through Pakistan into Afghanistan and is a home to several extremist groups, including the Pakistani Taliban.  

Also Monday, Pakistani authorities arrested at least 11 people in the southern port city of Karachi in connection with targeted political killings.

Interior Minister Rehman Malik said more than 43 people have been killed in Karachi in 18 days.  He said the assassinations were part of a "conspiracy against Pakistan to destabilize it."  He did not elaborate.

Authorities have said those killed included activists with the Muttahida Qaumi Movement, or MQM, a partner in Sindh province's ruling coalition.

The MQM, which largely represents the Urdu-speaking community, and the rival Awami National Party, representing ethnic Pashtuns, are blaming each other for the latest political violence.

Some information for this report was provided by AP, AFP and Reuters.