A senior US military official has told NBC News that the Nato soldier killed by an Afghan Local Police officer in the east of the country was an American.
Earlier two British troops were shot dead by an Afghan soldier.
16 Nato servicemen have been killed by Afghan Security forces so far this year.
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The Duchess of Cambridge has been on maternity leave since the week before Easter.
The Duke and Duchess of Cambrige have been together for 15 years and will have three children under the age of four.
News of William and Kate’s third child was announced on September 4 and the duchess went on maternity leave on March 22.