US Defence Secretary Mark Esper says under current plans all US troops leaving Syria will go to western Iraq and the American military will continue to conduct operations against the Islamic State group to prevent its resurgence.

His comments were the first to specifically lay out where American troops will go as they shift from Syria and what the counter-IS fight could look like.

Esper, who arrived in the Afghan capital on Sunday, did not rule out the idea that the US forces would conduct counter-terrorism missions from Iraq into Syria. But he told reporters travelling with him that those details will be worked out over time.

Esper, who flew overnight to Afghanistan, said he has spoken to his Iraqi counterpart about the plan to shift about 1,000 troops leaving Syria into western Iraq.