The Austrian Chancellor, Viktor Klima, said the decision to start up the first reactor at the Mochovce nuclear plant was an unfriendly and highly irresponsible act. He said Austria was considering withdrawing its ambassador.

Officials at the Mochovce nuclear power plant - about 180km east of Vienna - announced on Monday that they have started up the first reactor, which is expected to become fully operational by mid-July.

Three more reactors at the plant are already in an advanced state of construction.

The International Atomic Energy Authority is to oversee talks between Slovakia and Austria

The Soviet-designed power station has been upgraded using western technology, with the intention of making it safer. However, international experts who inspected it in May expressed reservations about the safety of the new features as well as of the original design.

The Austrian leader of the team of experts said he regretted the decision to activate the plant.

Such criticism has been rejected as politically motivated by the Slovak energy utility responsible for running Mochovce, which claims instead that the plant is a successful example of east-west co-operation in nuclear energy.

Further talks are expected between the Austrian and Slovak governments under the auspices of the International Atomic Energy Authority in Vienna.

The Austrian government has long been opposed to the plant and has warned that bringing it into operation could damage Slovakia's hopes of joining the European Union. But Slovakia insists that the plant is safe and says it is vital to meeting the country's energy needs.

The environmental group, Greenpeace, has called for a firm response from the Austrian government and the European Union.

Controversy has surrounded the power plant at Mochovce from the moment construction began in the early 1980s.

Work was halted in the early 1990s when the project ran into financial problems. A Franco-German plan to complete it collapsed when the European Bank for Reconstruction and Development withdrew in 1996.