A Belarusian journalist who specializes in defense has been missing for several days, his colleagues said on Thursday, raising fears he has been detained by security forces.

Journalist Alexander Alesin "has not come to work for several days now," Irina Krylovich, deputy editor of Belarusians and the Market independent newspaper where the journalist works in Minsk, told Agence France-Presse.

Lithuanian prosecutors on Wednesday charged a military medic with spying for neighboring Belarus, in the second espionage case involving the Russian ally in less than a month.

"The suspect accused of espionage worked for the Belarusian secret service," state prosecutor Raimondas Petrauskas told reporters.

U.S. troops will remain in Poland and the Baltic states through 2015 "to deter Russian aggression," the top U.S. commander for Europe said on Sunday.

"There are going to be U.S. army forces here in Lithuania as well as Estonia and Latvia and Poland for as long as it's required to deter Russian aggression," said Lt. General Ben Hodges, commander of U.S. Army Europe.

Lithuania on Monday announced a new rapid reaction force designed to meet what it termed new unconventional security threats highlighted by the Ukraine crisis.

The Baltic nation's top general said 2,500 troops will be on high alert as of November to counteract so-called "hybrid warfare" involving unconventional attacks by unmarked combatants, like those in eastern Ukraine.

Russia is acting like a "terrorist" in Ukraine and in its intimidation of other neighbors, Lithuania's President Dalia Grybauskaite said in an interview released Thursday.

"I think that Russia is terrorizing its neighbors and using terrorist methods," she told the Washington Post newspaper, replying "Yes" when asked whether she thought both Russia and the Islamic State (IS) group, which is sweeping across Syria and Iraq, were terrorists.

Lithuania, Poland and Ukraine on Friday signed an agreement to form a joint brigade intended to modernize the Ukrainian army confronting pro-Kremlin separatists in the country's east.

"This shows our deep commitment to boosting the security and defense capabilities of our region, where the situation has become dangerous," Polish President Bronislaw Komorowski said after defense ministers from the three countries inked the accord in Warsaw.

NATO will set up regional centers in a handful of Eastern European countries, Lithuania said on Friday as top military commanders met in the Baltic state's capital.

Defense chiefs from the alliance's 28 nations are taking part in a three-day meeting in Vilnius along with NATO top commander General Philip Breedlove amid anxiety in the region over Russia's actions in Ukraine.

In thick pine forests hidden in the remote wilderness of eastern Lithuania, young professionals are ditching their suits and ties for camouflage gear, and swapping iPads for rifles.

Lithuanian President Dalia Grybauskaite on Saturday warned Russia was practically at war with Europe and urged tougher sanctions against Moscow.

"Russia is at war against Ukraine and that is against a country which wants to be part of Europe. Russia is practically in war against Europe," said Grybauskaite, one of Moscow's harshest critics, as she arrived for an EU summit.

Lithuania's Foreign Minister Linas Linkevicius said Friday that the country's honorary consul in eastern Ukraine's Lugansk has been kidnapped and killed by the pro-Russian rebels there.

"With deep sorrow just learned about Lithuania's honorary consul in Lugansk Mr. Mykola Zelenec kidnapped and brutally killed by terrorists there," Linkevicius, who is on a visit to Kiev, wrote on his Twitter blog.