By Luke Baker

KHOST, Afghanistan (Reuters) - The announcement over the military address system echoed ominously across the U.S. base in southeastern Afghanistan, freezing soldiers in their tracks.

"Attention on the FOB! Attention on the FOB!" the voice boomed in the darkness, addressing Forward Operating Base Salerno, a camp 20 km (13 miles) from the Pakistan border.

"Emergency blood drive, O positive. Emergency blood drive, O positive," the voice bellowed, steady yet urgent.

From various corners of the camp, soldiers slung assault rifles over their shoulders and began to run towards the combat support hospital, near the air strip. Casualties were in.

It may be barely perceptible at this stage, but slowly and surely attacks by the Taliban and militants allied to them appear to be picking up along Afghanistan's southeastern border.

U.S. commanders have been braced for a "spring offensive", a pick-up in violence tied to the season, when warmer weather allows the Taliban to work their way over the mountains from hideouts in northwestern Pakistan and into Afghanistan.