US troops returning from West Africa are being quarantined at a base in Italy as a precaution to prevent the potential spread of the Ebola virus, the Pentagon said Monday.

The outgoing commander of the US military mission in Liberia, Major General Darryl Williams, along with 11 other members of his staff, were the first to undergo the isolation measures, which will last up to 21 days, a Pentagon spokesman told reporters.

"Out of an abundance of caution, the army directed a small number of personnel, about a dozen, that recently returned to Italy, to be monitored in a separate location at their home station of Vicenza," Colonel Steven Warren said.

But he added: "None of these individuals have shown any symptoms of exposure."

The general and the other soldiers were assigned to a separate building at the base in Italy and were being monitored by a medical team.

The decision was taken by the US Army but "there was no specific event or incident that triggered all of this," Warren said.

Dozens of other soldiers due to fly back from Liberia and Senegal also will be placed under isolation and subject to "enhanced monitoring" for a period up to 21 days, Warren said.

Officials initially insisted the move was not a "quarantine," but later acknowledged the isolation steps did amount to such a measure.

The US Army's decision to isolate the troops appeared to contradict earlier Pentagon policy pronouncements that indicated service members without symptoms could return to work.

A Department of Defense memo issued October 10 states: "Asymptomatic DoD personnel who meet the criteria for 'no known exposure' will return to work and continue twice-a-day unit monitoring."

The White House said the Defense Department was reviewing its policy and that the Army's decision did not apply to other military branches.

"I know that there was this decision that was made by one commanding officer in the Department of Defense, but it does not reflect a department-wide policy that I understand is still under development," spokesman Josh Earnest told reporters.

There are now 700 US troops in West Africa, including nearly 600 in Liberia and 100 in Senegal, helping with international efforts to fight the spread of Ebola.

The force is due to grow to at least 3,200 troops in the coming weeks and possibly as large as 3,900.

The US military team has set up mobile labs to test for the virus, built a 25-bed hospital for health workers and is building Ebola treatment units.

West Africa is the epicenter of the Ebola outbreak that has claimed the lives of nearly 5,000 people.