Sullivan said the United States has been pursuing transparency surrounding the situation to ensure that Russia is not provided the opportunity to "spring something on Ukraine or the world."
"We are going to make sure that we are laying out for the world what we see as transparently and plainly as we possibly can and share that information as widely as we can," he said. "That's what we've done. That's what we'll continue to do."
U.S. President Joe Biden is set to speak with Zelensky by phone on Sunday, Zelensky's spokesman Sergii Nykyforov confirmed via Facebook.
The call comes a day after Biden warned Russian President Vladimir Putin on a phone call of "swift and severe costs" if Russia moves forward with an invasion against Ukraine.
German Chancellor Olaf Scholz is also set to meet with Zelensky in Kiev on Monday and Putin in Moscow on Tuesday.
Last week, French President Emmanuel Macron said
he'd received assurance from Putin that he would not escalate the tensions.