"This aggressive striving for power with regard to Ukraine represents not only a danger for the Ukrainian state — other parts of eastern Europe are also in danger," she told German daily Der Tagesspiegel.
"Should Putin continue his attack on our country after the annexation of Crimea, I would call on the leaders of the democratic world to use the strongest measures to stop this aggressor."
She said the West should under no circumstances accept the result of referendum in Crimea on breaking away from Ukraine to join Russia.
"It is without precedent in recent history and such methods used by Russia in Crimea were no longer considered possible by many in the West," she said.
Tymoshenko, a leader of the pro-Western Orange Revolution in 2004, has been receiving treatment at Berlin's Charite university hospital since her arrival in the German capital this month.
The 53-year-old was freed from prison on February 23rd, having served three years of a seven-year sentence for abuse of power, a charge she has always denied.
Der Tagesspiegel said she is in constant telephone contact with allies in Kiev and received a visit from former Russian oil tycoon and Kremlin critic Mikhail Khodorkovsky, who spent a decade in prison before his release in December.
Khodorkovsky, who has applied for residency in Switzerland, visited Kiev last week and slammed Moscow's interference in Ukraine.