OTTAWA - Prime Minister Stephen Harper has arrived back in Ottawa after an overseas trip.

The prime minister paid a surprise visit to Afghanistan on Thursday to rally Canadian troops and showcase their mission's shifting priorities with the addition of some 17,000 more U.S. soldiers into the country's south this summer.

Harper also visited the Dahla dam, one of Canada's signature reconstruction projects, in the fertile Arghandab valley that will provide drinking water to much of the Kandahar region when it's completed.

He pledged an additional $2 million in Canadian aid that would go through UNICEF to help educate an estimated 18,000 children in Kandahar.

The Afghan visit followed trade meetings Wednesday with European officials in Prague, in which Harper announced Canada and the European Union have formally launched negotiations toward a free trade agreement.

The prime minister had also announced in Prague on Wednesday agreements with the EU on air safety and air transport that could lead to lower prices for travel between Canada and Europe.