KIEV, Ukraine (AP) — Yulia Tymoshenko, Ukraine's former prime minister, urged the West on Wednesday to ramp up pressure on Russia to force it to withdraw troops from Crimea.

In an interview with The Associated Press two weeks after she was released from jail, Tymoshenko, 53, said the United States and Britain must engage directly with Russia and use "the most powerful tools" to ensure that Russian troops leave the Crimean Peninsula, which they have been occupying for nearly a week after the ouster of pro-Russian President Viktor Yanukovych.

Tymoshenko said that as the signatories of a 1994 treaty, which guarantees Ukraine's security in exchange for it giving up its Soviet-era nuclear weapons, the U.S. and Britain must now deal directly with Russia. She said Ukraine cannot enter any negotiations with Moscow while Russian troops are pointing guns at its soldiers.

"It is up to them (the U.S. and the UK) to choose the methods to stop the aggressor. But they must do it immediately," Tymoshenko said at her office in downtown Kiev. The West must do "everything that will stop the aggressor. Period."

Tymoshenko spent two-and-a-half years in jail on charges of abuse of office that the West condemned as politically motivated.

During the interview, she refused to say if she plans to enter Ukraine's May 25 presidential election. Although she now holds no formal post, she is believed to wield significant political influence since her closest ally, Oleksandr Turchynov, is the acting president.

Tymoshenko, who suffers from a back condition, walked slowly leaning on walking aids. But clad in an elegant grey jacket with her blond braid wrapped around her head in her trademark peasant style, she looked much better than two weeks ago. That's when Tymoshenko appeared on a stage in a large protest camp in the center of Kiev, sitting in a wheel chair and looking pale and worn out.

Tymoshenko called for a quick signing of a political and economic treaty with the European Union. When Yanukovych shelved it, that promoted the mass protests in Kiev that eventually led him to flee the country for Russia last month.

Once the Russia-Ukraine standoff is over, Russia's Black Sea Fleet, which is currently based in Crimea as part of a leasing agreement, must leave, said Tymoshenko.

"Today it is obvious that basically the Black Sea fleet has become the source of a war ... a ground for seizing our state," she said.

Tymoshenko added that Ukraine must not make any compromises to appease Russia. "We believe that the aggressor must leave without any conditions," she told AP.