Tillerson is expected in Kenya later this week
Mr Tillerson is set to hold talks with President Uhuru Kenyatta in his week-long visit to Kenya and four other African countries commencing Tuesday.
Donald Yamamoto, the acting assistant secretary of state for African affairs, noted in a press briefing that he and US Ambassador Robert Godec have spoken with Mr Odinga and other Nasa figures on numerous occasions.
He said US officials have had "behind the scenes" talks with the Opposition that have contributed to “robust relationships” between Mr Odinga and other figures in Nasa.
“We're not ignoring the opposition,” the acting head of the Africa Bureau added. “It plays a critical role”.
Mr Yamamoto argued that Tillerson's visit is aimed at having talks with the President Kenyatta's government.
He also endorsed the performance of Ambassador Godec despite calls from some Nasa MPs to have him recalled from Kenya. Foreign envoys were accused by Nasa of failing to address electoral injustice following last year's protracted polls.
Mr Yamamoto however noted that Washington's envoy to Nairobi is among the most senior US diplomats serving in Africa. He said Ambassador Godec stands as “a model of leadership and engagement.”
Mr Tillerson’s visit to Kenya and four other African countries is partly intended to make amends for President Trump’s reportedly vulgar remarks about the continent and to counter the perception that Africa now matters little to US policymakers.