Russia examines 1991 recognition of Baltic independence
The Russian chief prosecutor's office is to examine whether the Soviet Union acted legally when it recognised the Baltic states' independence in 1991.
The investigation was described as an "absurd provocation" by Lithuania's Foreign Minister Linas Linkevicius.
Estonia, Latvia and Lithuania were occupied by Soviet communist forces in 1940. The USSR broke up in 1991.
Last week Russia's chief prosecutor declared illegal the transfer of Crimea from Russia to Ukraine in 1954.
At the time Russia and Ukraine were republics of the USSR, under communist leader Nikita Khrushchev.
Russia's annexation of Crimea in March 2014 was condemned internationally. Ethnic Russians there voted to rejoin Russia, in a highly controversial referendum.
There are large ethnic Russian minorities in Estonia and Latvia, while Lithuania has a smaller ethnic Russian minority.
A source at the prosecutor's office, quoted by Russia's Interfax news agency, said the investigation into the Baltic states' independence followed a request from two parliamentary deputies.
In their letter, MPs Yevgeny Fyodorov and Anton Romanov, of President Vladimir Putin's United Russia party, said the 1991 decision to recognise Baltic independence had been taken "by an unconstitutional body".
The three Baltic states joined the EU and Nato in 2004. In recent years Russia has viewed that as a hostile challenge to its security interests.
Russian-Baltic tensions have been rising since the Crimea annexation and the outbreak of fighting in eastern Ukraine in April 2014. Heavily armed pro-Russian separatists there are clashing daily with Ukrainian government troops.
Nato has stepped up its presence in the Baltic states, responding to massive Russian military exercises, including heightened Russian air force activity in the Baltic.
Reacting to the Russian prosecutor's move, Lithuania's foreign minister called it "a provocation to say the least" and "legally, morally and politically absurd".