Tillerson will be replaced by a lower-level official at the NATO meeting on April 5-6.
Tillerson will be replaced by a lower-level official at the NATO meeting on April 5-6, a development that will likely fuel new concerns about US President Donald Trump's commitment to the alliance as he pushes for better ties with Moscow.
"Tom Shannon (the US Under Secretary of State) will represent the United States at the NATO Foreign Ministerial," the US official told AFP on condition of anonymity.
Tillerson will meet most of the foreign ministers from the 28-nation military alliance at a meeting in Washington this week, the official added.
"He has already met with officials from Ukraine. After these consultations and meetings, in April he will travel to a meeting of the G7 in Italy and then on to meetings in Russia," the official said.
Under president Barack Obama's administration, NATO sought to shore up support for the pro-western government in Kiev after Russia's annexation of Crimea and its support for an uprising in eastern Ukraine.
But during his election campaign, Trump expressed admiration for Russian President Vladimir Putin and said he would seek better ties with his government, while in January he dismissed NATO as "obsolete."
Visiting NATO headquarters in Brussels last month, US Vice President Mike Pence sought to reassure the allies about his government's commitment to them, particularly as the Baltic states and Poland fear Russian meddling in their affairs.
But Pence said that Trump expects NATO allies to make real progress by the end of this year toward meeting the increased defence spending target agreed by the alliance.
NATO chief Jens Stoltenberg travelled to Washington on Monday for the first time since Trump was elected, and will hold talks with members of the US-led coalition working to defeat the Islamic State group.
The Trump administration has sought to involve NATO more in fighting the jihadists.