VILNIUS (AFP) - - Lithuania is facing what appears to be a concerted cyber assault by pro-Russian hackers amid a slide in relations between the Baltic state and its Soviet-era master, a top regulator said Monday.

The national communication regulator's office said that some 300 websites, including those of public institutions such as the national ethics body and the securities and exchange commission, as well as a string of companies, had found themselves under attack.

The sites' content was replaced with images of the red flag of the Soviet Union, alongside anti-Lithuanian slogans.

Lithuania broke free from the Soviet Union in 1991 after five decades of rule from Moscow, and joined the European Union and NATO in 2004.

Its relations with Russia have been on a downturn in recent months, after Vilnius adopted a harder line in a string of long-running bilateral disputes.

In April, Lithuania unilaterally blocked EU attempts to kick off talks on a new partnership deal with energy-rich Russia, to replace the old deal signed in 1997 when Moscow was still reeling from the Soviet collapse.

Any of the EU's 27 member states has the right to block talks between the union and other countries if it feels its national interests are being sidelined.

Vilnius demanded that a number of sensitive issues be spelled out in the EU negotiating mandate, including Russia's active cooperation over energy supplies.

Russian oil supplies via the "Druzhba" pipeline to Lithuania were cut off in 2006, supposedly because of maintenance work, after Lithuania decided to sell its only oil refinery -- Mazeikiu Nafta -- to a Polish group instead of a Russian one.

Lithuania also demanded that the EU push Russia to bring to justice members of the Soviet security forces involved in bloody crackdowns on the independence movement.

Fourteen civilians were killed in Vilnius and hundreds were injured in January 13, 1991, and seven Lithuanian border guards and policemen were gunned down on July 31, 1991.

Vilnius also wants Russia to respect its international obligation to compensate Lithuanians who were deported to the Soviet Gulag after Moscow took over the country during World War II.

Lithuania's fellow EU members convinced it to lift the veto in May, after pledging to include Vilnius' concerns in the negotiating mandate laid down for the European Commission, the EU's executive body which steers such talks.

Several weeks ago, Lithuania angered Russia further still when lawmakers voted to ban the public display of both Nazi German and Soviet symbols, including flags.

For many Russians, forbidding the Soviet banner is an affront to the memory of Red Army soldiers who died fighting the occupying Germans in Lithuania during World War II.

For many Lithuanians, however, Soviet symbols are a painful reminder of their decades under Moscow's thumb.

Lithuania's fellow Baltic state Estonia was also hit by massive cyber attacks last year after the authorities decided to move a Red Army memorial from the centre of the capital Tallinn.