Washington (CNN) The US ambassador to Afghanistan will leave his post Monday, a State Department official said, a "long-planned" but notable departure that comes as attempts at peace talks with the Taliban have yet to yield results and a crisis between the US and Iran threatens security and stability in the broader region.

The departure of John Bass, who had served in Kabul since December 2017, is "part of the normal rotation cycle," the State Department official said.

In a farewell message, the career diplomat said his "earnest hope is for leaders and citizens across this country to find strength in unity, put aside their differences, and work together to negotiate a political settlement with the Taliban."

"Afghans and this beautiful country deserve nothing less," Bass wrote on Twitter.

The State Department official said Monday that the US Embassy in Kabul would be led by Amb. Ross Wilson, a former foreign service officer who served for decades in multiple administrations. Wilson is expected to arrive in Afghanistan "soon," the official said, and deputy chief of mission Karen Decker will serve as the charge d'affaires until then. The White House has yet to nominate a permanent replacement for Bass.

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