LONDON - Britain will raise troop numbers in Afghanistan to 9,500, Prime Minister Gordon Brown said on Wednesday, an increase of 500, providing key conditions were met.

Worsening violence in the eight-year, U.S.-led war against the Taliban has led to calls for a change of strategy including the option of sending more troops to bolster security.

There are more than 100,000 Western troops serving in Afghanistan, of whom 65,000 are U.S. troops. The number of U.S. troops already is due to increase to 68,000 later this year.

Brown said Britain would send the additional troops as long as the Afghan government sped up the training of its own forces and the correct military equipment was available.

He said the British increase also had to be part of an agreed approach with other military forces in Afghanistan, with all countries bearing their fair share.

I've agreed in principle to a new British force level of 9,500 which will be put into effect once these conditions are met, Brown told parliament.

U.S. President Barack Obama is considering a military recommendation to boost his force with a further 40,000 troops next year.

Britain's force is the second biggest contribution to the NATO mission.

(Reporting by Tim Castle and Matt Falloon; Editing by Matthew Jones)