UPDATE 2-Sarkozy flying to Afghanistan after French deaths

(Adds reaction from U.S., Britain, death of NGO worker)

By James Mackenzie

PARIS, Aug 19 (Reuters) - President Nicolas Sarkozy prepared to fly to Afghanistan on Tuesday after 10 French soldiers were killed and 21 wounded in a clash with Taliban insurgents, and declared France remained committed to its mission there.

"My determination is intact. France is determined to continue the struggle against terrorism for democracy and freedom. The cause is just. It is an honour for France and its army to defend it," Sarkozy said in a statement.

The soldiers, from three elite paratroop and marine regiments, were killed in a fierce battle when their unit was ambushed on a reconnaissance mission in the Sarobi district, some 60 km (40 miles) east of Kabul on Monday.

The loss was the worst suffered by the French army in a single incident since 58 paratroops were killed by a suicide bomber in Lebanon in 1983 and the worst in combat with enemy forces since the Algerian war that ended in 1962.

The United States and Britain expressed sympathy and solidarity with France while stressing that the effort in Afghanistan continued.

"It's a difficult time for France, but the French have issued a statement saying they plan to stay the course in Afghanistan," said U.S. State Department spokesman Robert Wood.

British Prime Minister Gordon Brown said: "Theirs was a sacrifice not just for France but for all of us who want to help the Afghan people build a better future for themselves."