The U.S. troops have arrived in the Ebola epidemic zone, and they're not carrying guns. Instead, clipped to the belt loops of their military uniforms are tiny plastic bottles of hand sanitizer.

The soldiers grinned sheepishly as they lifted their shirts to show the personal hygiene bottles on their belts, designed to help protect them from the Ebola virus. But some admitted that their families are just as worried about the dangers of their deployment in the Ebola zone as they were about the risks of their earlier stints in the shooting wars of Afghanistan and Iraq.

Several dozen U.S. troops have arrived at Roberts International Airport, just outside Liberia's capital, Monrovia. They are the vanguard of the Pentagon's planned deployment of 3,000 troops in West Africa to fight the Ebola outbreak.

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On Saturday, four U.S. naval engineers began the first phase of construction of a 25-bed field hospital for health workers in Liberia who catch the Ebola virus. A heavy grader was smoothing the damp weedy soil at an empty field near the international airport as the project was launched.

The Pentagon had announced plans for the $22-million (U.S.) hospital on Sept. 8, two days after U.S. President Barack Obama pledged to send military help to the Ebola-afflicted countries.

The hospital construction began 19 days later – a relatively speedy process for any U.S. government project, according to Washington-based analysts.

Meanwhile, other U.S. troops were evaluating the runway at the international airport, assessing its weaknesses and how to strengthen the tarmac for an expected surge of cargo planes and aid flights in the coming weeks.

The airport is remarkably quiet these days. Most airlines have cancelled all commercial flights to Liberia because of Ebola fears. Only two airlines continue to fly into Liberia. A few cargo flights are landing, with Ebola aid supplies from United Nations agencies and other sources, but those flights are still relatively infrequent, despite the massive global publicity about the Ebola crisis.

On Saturday, only two parked airplanes could be seen in the entire airport, but a small joint force of U.S. army and air-force troops were preparing for future flights.

Humanitarian duties have become a bigger part of the mandate of the U.S. military in recent years, including deployments in Haiti after the devastating earthquake of 2010 and in Indonesia and Thailand after the 2004 tsunami.

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"We're excited, we were actually thrilled to be chosen to come here and help out a country that's in need, in big need, especially with this Ebola virus going on," said Petty Officer Richard Brown, a member of the "Seabees" – the U.S. Navy Construction Battalion.

A U.S. military spokesman, Major Jason Brown, said there is no fixed date for finishing the constructio, partly because the heavy downpours of the Liberian rainy season could be an obstacle.

"We're moving faster than I've ever seen the military move on anything before," he said. "We're already breaking ground on a construction project when we had nothing here before."