WASHINGTON — They are being heralded as a key part of President Trump’s new strategy to resolve the nearly 17-year war in Afghanistan. But their training has been cut short by months, and units are still short-staffed, as some of the estimated 1,000 additional military advisers prepare to arrive in Afghanistan in time for the spring fighting season, officials said.

The Army soldiers are deploying as the Pentagon begins shifting resources from the fight against the Islamic State in Iraq and Syria back to Afghanistan. As part of its new assault against an increasingly tenacious Taliban, the Trump administration is planning to send some of the advisers back to small bases scattered across rural parts of the country to help train Afghan forces.

The new brigade of advisers was formed in August and is based at Fort Benning, Ga. Two military officials said its leaders were still trying to ensure that each small team had enough soldiers to train Afghans.

One of those officials, and an additional one, said that the advisers’ brigade was supposed to have around a year of training before deploying. Advisers in the new brigade are expected to begin deploying by early spring — roughly eight months after the brigade was created.