The senior Nato general in Afghanistan has said that many of the 17,000 reinforcements ordered by President Barack Obama will go to the heart of British-patrolled Helmand province.
Outlining for the first time where the new troops will deploy, Gen David McKiernan said the 2nd Marine Expeditionary Brigade will be spread from the provincial capital of Lashkar Gah south towards Garmsir, as well as in the east of neighbouring Farah province.
The brigade, featuring armoured vehicles and aircraft, is designed to be highly mobile.
In all, the unit is expected to make up 10,000 men, compared to 5,700 British troops now in Helmand, and will arrive in May or June.
The US is building a major extension to Camp Bastion, outside Lashkar Gah, where Britain has led military and civilian efforts in the province for three years.
Gen McKiernan has dismissed criticism by some in the US forces about British performance, but has acknowledged Nato-led forces have reached "stalemate".
After three summers of bloody fighting, British commanders admit they do not have enough men to hold areas after clearing them of insurgents.
"It's not that the British are being sidelined, we are being assisted," said Capt Mark Durkin, spokesman for the Nato-led coalition.
"General McKiernan has said on a number of occasions the British have done a fantastic job." Garmsir is currently patrolled by the Light Dragoons battle group.
While able to consistently beat insurgents in battle, British commanders struggle to hold areas beyond the province's district centres. They hope the American marines will allow them to link up these pockets of security.
Gen McKiernan also said a US army brigade of Stryker armoured vehicles will be deployed around Kandahr province, in July and August, in the eastern districts around Spin Boldak and northern regions around Arghandab, Khakrez and Shah Wali Kot.