Embattled Ukraine lacks "the power or the possibility" to stand up to Russia at present, Ukraine's former prime minister told CNBC.
Yulia Tymoshenko, who led the government in 2005 and again from 2007-2010, said the U.S. and the U.K. needed to help Ukraine to combat any future incursion by Russia.
"Of course, Ukraine at this point doesn't have the power or the possibility to face Russia," she said.
"Any kind of interrelations with Russia should be four-part negotiations, so, not just between the Russia and Ukraine, but between the countries that signed the Budapest Memorandum (a security agreement signed by Russia, the U.K. and the U.S. in 1994)."
Ukraine has remained on the brink of crisis since the overthrow of the pro-Russian president Viktor Yanukovych in February. In early March, Russia annexed Crimea in the south of the country. Since then there has been fighting between pro-Russian militias and Ukrainian forces in the country's industrial heartland in the east.
As part of Yanukovych's ouster, Tymoshenko was released from prison, where she had been held for three years on charges of embezzlement and abuse of power.
Russia's aggression has led both the U.S. and the European Union (EU) to hit it with an escalating series of sanctions. These include visa bans and asset freezes on companies and officials in both Ukraine and Russia with links to President Vladimir Putin.
Tymoshenko called for sanctions to be increased still further. "The democratic world has to continue to increase sanctions and to continue with stopping Putin's aggression until the occupation of Crimea is over," she told CNBC.