DESERT AIRSTRIP - U.S. marines camped in the southern Afghan desert passed a quiet night devoted to bringing in more troops and equipment to build up the strength of forces at the airstrip they seized on Sunday.

The marines would not say how many troops were on the ground by Wednesday morning, but helicopters and KC130 Hercules planes arrived throughout the night.

Many of those on the ground were spread out in outlying defensive positions, making it hard to gauge the total numbers.

"The Marines have clearly strengthened their position," said Captain David Romley, spokesman for the 15th Marine expeditionary unit which took the airstrip at the weekend.

"There was no sort of engagement on our perimeter" however, we continue to flow in additional personnel, equipment and ordinance."

On Monday, Cobra attack helicopters fired on a convoy of military vehicles that were heading towards the airstrip, whose location has not been released because of security. It is described by the Marines as within striking distance of Kandahar.

Marines were carrying out extensive reconnaissance by air and land around the airstrip and officials warned media organisations in Afghanistan against trying to find the base as all vehicles in the area were coming under suspicion.

"We are advising media not to try to find the Marine operating base here because of security concerns and the inability to tell friend from foe," said Captain Stuart Upton, spokesman for Taskforce 58, which is carrying out Operation Swift Freedom. Nampa-Reuters