The former Georgian President, Mikheil Saakashvili, says he will go back to Ukraine to mobilise his supporters there, in defiance of Kiev's decision to strip him of Ukrainian citizenship.
Mr Saakashvili moved to Ukraine in 2013 and served as governor of Odessa under President Petro Poroshenko. Both have championed pro-Western market reforms.
But the two men have since fallen out.
Last year Mr Saakashvili said corrupt oligarchs close to Mr Poroshenko were blocking the reforms.
Speaking to the BBC's Newshour programme from New York, Mr Saakashvili, who heads an opposition party in Ukraine, said Mr Poroshenko was trying to remove his most serious opponent.
However, according to opinion polls, Mr Saakashvili's Movement of New Forces (Rukh) has less than 2% support among Ukrainians.
Accused of corruption in his native Georgia, he lost his Georgian citizenship in 2015 after becoming a Ukrainian national. Mr Saakashvili rejects the Georgian corruption charges as politically motivated.
According to the Ukrainian authorities, he put false information on his citizenship registration form in 2015.
He told the BBC that he would strive to "get rid of the old corrupt elite" in Ukraine. But he denied any ambition to become Ukrainian president.
Mr Saakashvili said corrupt Ukrainian oligarchs were annoyed by his campaign to clean up the economy, which has been stagnating for years.
"I'll do my best to return to Ukraine," he said, calling President Poroshenko "extremely unpopular".
The former Georgian president has close ties with some US politicians, but said he would not seek asylum in the US, despite the risk now of being stateless.
Last year, he resigned as the Odessa governor, expressing fury at rampant corruption, and launched his political party in Ukraine.
In a statement (in Ukrainian) on Wednesday evening, Ukraine's migration service said that "according to the Constitution of Ukraine, the president takes decisions on who is stripped of Ukrainian citizenship based on the conclusions of the citizenship commission".
It did not provide the exact reason, but stated that this could be done if a Ukrainian national acquired citizenship of another country or submitted false documents.