"If the regime and its backers think they can test boundaries, diminish their compliance in certain areas, or act in ways that call into question their commitment to the cessation -- without serious consequences for the progress we have made -- they are mistaken," Kerry said after talks with his European allies in Paris.
Kerry described comments by his Syrian counterpart that removing President Assad would cross a "red line" as an attempt to disrupt the peace process.
He said Foreign Minister Walid Muallem's remarks were "clearly trying to disrupt the process... (he was) clearly trying to send a message of deterrence to others".
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"But the fact is (Assad's) strongest sponsors Russia and Iran have both adopted... an approach which dictates that there must be a political transition and that we must have a presidential election at some time," Kerry said on the eve of a new round of peace talks.
French Foreign Minister Jean-Marc Ayrault also said Muallem's comments were a "provocation".
Kerry also said 600 fighters from the Islamic State group had been killed in Syria in the past three weeks.
"In Syria, over the last three weeks alone, Daesh has lost 3,000 square kilometres (1,160 square miles) and 600 fighters," he said, using another name for Islamic State.