U.S. troops arrived Saturday in Lithuania, part of a U.S. contingent of 600 sent to the region to reassure NATO allies amid the escalating Ukraine crisis.

Lithuanian President Dalia Grybauskaite hailed the arrival of American forces as a "deterrence measure" in the region where alarm has risen over Russia's actions in Ukraine.

Lithuania must overturn a lifetime ban that stops impeached ex-president Rolandas Paksas from seeking political office because it violates his rights, a U.N. watchdog body said Wednesday.

Paksas, the first ever European head of state to be ousted by impeachment, has spent a decade battling the lifetime ban and filed a complaint with the U.N. Human Rights Committee.

Europe must stand united as Russia challenges the continent's resolve over the Ukrainian crisis unfolding on its eastern borders, Lithuania's top diplomat told Agence France Presse on Friday.

"Russia is challenging our unity, our way to handle the crisis," Foreign Minister Linas Linkevicius said in an interview on the sidelines of a meeting with his European Union counterparts.

U.S. Vice President Joe Biden on Wednesday said Washington could send forces to the Baltic states to reassure ex-Soviet NATO allies rattled by Russia's takeover of Crimea.

"We are exploring a number of additional steps to increase the pace and scope of our military cooperation, including rotating U.S. forces in the Baltic region to conduct ground and naval exercises and training missions," Biden said during a visit to Lithuania, after a first stop in Poland.

Lithuania on Monday alleged that a Russian spy worked undercover at Moscow's embassy in Vilnius, offering bribes for information on EU ties with former Soviet states.

According to the claims from the Lithuanian State Security Department -- the Baltic nation's intelligence agency -- diplomat Valery Katula worked for Russia's GRU military intelligence service.

The United States will on Thursday send six additional F-15 fighter jets to step up NATO's air patrols over the Baltic states, mission host Lithuania said as West-Russia tensions simmered over Ukraine.

"I have had confirmation that the air police missions will be reinforced by six additional F-15 fighters," Defense Minister Juozas Olekas told AFP.

Anxious Western powers voiced fear over the growing Crimea crisis Thursday, warning Russia not to escalate tensions and telling pro-Moscow separatists they were playing a "dangerous game."

After dozens of pro-Russian gunmen seized government buildings in the volatile peninsula -- raising fears of a full-blown regional conflict -- NATO leader Anders Fogh Rasmussen tweeted his concern and pleaded with Moscow "not to take any action that can escalate tension or create misunderstanding."

Lithuanian prosecutors said Thursday they have opened an investigation into claims that a Saudi terror suspect was held in an alleged secret CIA jail in the Baltic state.

The probe concerns the "possible illegal transportation of persons across the state border", Vilnius prosecutors said in a statement.

Lithuania on Tuesday confirmed signs of torture on a prominent Ukrainian anti-government activist and urged an independent probe into the crime which could breach a related United Nations convention.

"Dmytro Bulatov...has clear signs of long term torture and cruel treatment on his body," a Lithuanian foreign ministry said in a statement.

Lithuania said Friday it will provide free medical assistance to Ukrainians injured in violent protests in Kiev in a show of support for the pro-EU opposition.

The first group will include around 10 wounded protesters and will arrive in the Baltic state on Monday for treatment and physiotherapy, Lithuanian presidential adviser Jovita Neliupsiene told Agence France Presse.