Honda has said that it is staying in Formula 1 with McLaren next year.
In 2017, speculation has been rife that the Anglo-Japanese works collaboration will end after three hapless years.
More recent rumours say the Sauber customer deal for 2018 may also be off, triggering suggestions that Honda could be out of F1 completely.
However, Yamamoto did not deny that "talks" between McLaren and Honda are taking place.
"There is a contract between us and the premise of the talks with them is to continue," he said. "We exclude the possibility that McLaren will even temporarily use the power unit of another manufacturer.
"As for Sauber, we had very good negotiations with Monisha Kaltenborn and were moving in the right direction, but the new leadership changed course and now we are negotiating."
Frederic Vasseur took over at the Swiss team earlier this month following Kaltenborn's exit.