Tymoshenko, 53, said Tuesday she was concerned about the regional balance of power should Russia continue to rattle its sabers.
"When Russia is allowed to take away Crimea then the world will change," she told CNN. "Then not only life and politics in Ukraine will change, politics and life will change practically everywhere in the world."
Russian President Vladimir Putin said Tuesday he questioned the legitimacy of Ukraine's new leaders, but said there are no plans for an invasion.
"The use of troops is a last resort," he was quoted as saying by state-backed news agency RIA Novosti. "There is no necessity to do it at the moment, though that option is on the table."
Tymoshenko, an influential opposition leader in Ukraine, was freed from prison last month. She was sentenced by her rival, ousted President Viktor Yanukovych, in 2011 on corruption charges stemming from an energy deal with Russian energy company Gazprom.
Ukraine's new prime minister, Arseniy Yatsenyuk, hails from Tymoshenko's Fatherland party.
There was no statement from Kerry or Nuland. Nuland, Kerry's former spokeswoman, was criticized last month for a gaffe she made on European relations with Ukraine.