The United States has begun reducing its military presence in Iraq under a deal signed with Baghdad last year, with troop levels set to be cut by about 20 percent as responsibility for countering the Islamic State (IS) shifts to Iraqi forces, officials confirmed Wednesday.

Pentagon spokesman Sean Parnell said the reduction reflects the “combined success in fighting ISIS” and marks the next phase in Washington’s long-running military engagement in Iraq.

A senior US defense official said the drawdown will consolidate troops largely at a base in Irbil, in Iraq’s northern Kurdish region, with fewer than 2,000 service members expected to remain once the process is completed. That is down from just over 2,500 at present, and far below the roughly 20,000 troops deployed a decade ago.

The official, speaking on condition of anonymity, said Iraqi forces trained by the US over the past decade are now capable of leading operations against IS, though the group still stages deadly attacks in both Iraq and Syria.

Iraqi officials also confirmed that American forces have already begun leaving positions in Baghdad and Ain al-Asad airbase in western Iraq, leaving only “a very small number of advisers” at the joint command.

The shift in Iraq comes months after the Trump administration pulled about 600 troops from Syria, leaving fewer than 1,000 to support Kurdish partners there.

Prime Minister Mohammed Shia al-Sudani told AP in July that the two governments will meet before the year’s end to finalize the terms of a new bilateral security arrangement.

Washington has pledged close coordination with Baghdad and coalition partners to ensure what it calls a “responsible transition.”