The majority of the 17,000 extra U.S. troops being sent to fight a growing Taliban-led insurgency in southern and western Afghanistan should be on the ground by mid-July, the U.S. military said on Sunday.

A further 4,000 troops are arriving to train Afghan security forces and they will be deployed by August.

Washington pledged to send 21,000 additional troops to Afghanistan to reinforce security ahead of presidential elections scheduled for August 20 and to support NATO-led troops which have struggled to fight an escalating insurgency there.

"10,000 Marines are beginning to arrive now and will continue to arrive for the next month and a half or so and they will be principally located in Helmand but also in Farah," said Colonel Greg Julian, spokesman for U.S. forces.

Helmand province, in southern Afghanistan and Farah in the west are among the areas that have seen the fiercest fighting as insurgents gathered strength in recent years, despite the presence of a growing number of foreign troops.

"Everyday we are faced with suicide bombers, an increased number of improvised explosive devices," said General Zaher Azimy, spokesman for the Ministry of Defense.

"If the violence wasn''t increasing there would be no need for the presence of foreign forces and Afghan forces themselves could handle this issue."

Some 7,000 U.S. army troops are also being deployed to southern Kandahar province.