The Okhta Tower's designers describe it as 'twisting glass needle'
The UN's cultural watchdog has called on Russia to stop a 400m (1,312 ft) skyscraper being built in historic St Petersburg's city centre.
Unesco said the planned $2.4bn (£1.5bn) Okhta business centre tower would "damage the image of Russia".
Local authorities this week approved construction of the building, which will house offices of the state-controlled energy giant Gazprom.
St Petersburg's city centre is listed by Unesco as a world heritage site.
Unesco has warned the building of the skyscraper, which would tower over the city's Neva river and surrounding low-level buildings, could mean St Petersburg is added to the agency's list of endangered world heritage sites.
"We're hoping the (federal) decision to build it won't be taken," said Grigory Ordzhonikidze, the secretary-general of Unesco's Russian commission.
Twisting glass needle
The building's British designers describe the planned five-sided structure as a "396m-high twisting glass needle which echoes the spires across the city of St Petersburg".
St Petersburg usually has planning restrictions for buildings which are over 100m tall.
Announcing a relaxing of these rules for the skyscraper, Saint Petersburg Mayor Valentina Matviyenko said the Okhta project would bring more jobs and building projects to the city.
Visitors are impressed by the historic splendour of St Petersburg's buildings
Gazprom, a key export earner for Russia, sees construction of the business centre and skyscraper as a prestige project that would boost the international image of Russia's second city, which was founded by Peter the Great in 1703.
But conservationists say the glass and metal structure - which would be three times as tall as St Petersburg's current tallest building, the St Peter and Paul Cathedral - would ruin the city's ambience.
Opponents have taken legal action to block the project and clashed with police at consultation sessions held by city authorities.
It remains unclear whether Moscow's central government could intervene to veto the skyscraper, which would take several years to build.
St Petersburg is the home city of President Vladimir Putin and President Dmitry Medvedev, but has lagged behind Moscow in capitalising from the country's post-Soviet economic transformation.
Reflections and refraction
British architectural company RMJM, which was appointed to design the building in December 2006, welcomed St Petersburg's decision to relax its building height limitations for the project, which it called "a major step forward for the city".
RMJM says the tower's design was inspired by the concept of energy in water, "with the form of the building deriving its shape from the changing nature of water, ever-changing light, reflections and refraction".
Unesco's national commissions are set up by member states to co-ordinate the organisation's work with national governments and NGOs.
Russia's Hermitage Museum and the St Petersburg Union of Architects had previously voiced opposition to Gazprom's plan.