Russian Prime Minister Vladimir Putin on Monday promised a complete and objective investigation into the plane crash that killed Polish President Lech Kaczynski and other top officials.

Putin said at a government meeting that the causes of the crash that claimed all 96 lives aboard would be established.

Deputy Prime Minister Sergei Ivanov said the two black boxes discovered at the crash site have been confirmed as in fine technical conditions to allow further analysis.

Ivanov said flight data confirmed the crew received a warning about bad weather at Smolensk airport.

"It has been reliably confirmed that a warning of unfavorable weather at Severny Airport and recommendations to proceed to a backup aerodrome were not only transmitted to but also received by the crew," the Itar-Tass news agency quoted Ivanov as saying.

Russia observed an official day of mourning on Monday for the 96 victims killed in the plane crash near Smolensk in west Russia. The Soviet-made Tu-154 plane hit the top of trees as it attempted to land in thick fog at a Smolensk airport on Saturday morning.