Budapest - Parts of Hungary were on high flood alert on Thursday morning after a tributary of the Danube swelled to its highest level in over four decades, the local news agency MTI reported.

The Raab River rose to nearly 5 metres above its normal level in the western town of Szentgotthard, on Hungary's border with Austria.

This is 30 centimetres higher than a record set in 1965, when a major flood devastated large areas of western Hungary. Since then, however, flood defences have been built up along the river.

Nevertheless, trains between Hungary and Austria had to be cancelled as tracks were covered when streams broke their banks.

Austria has seen some of its heaviest rainfall in 50 years in recent days, causing widespread flooding.

Downriver in Hungary, towns along the Danube, including the capital Budapest, are now bracing themselves for possible flooding over the coming weekend.

The Danube had swelled by two metres in just 24 hours by Wednesday morning near Gyor, about 100 kilometres west of Budapest, prompting the authorities to issue a high-level flood alert.

Environment Minister Imre Szabo told reporters that the Danube is expected to peak at 7.6 metres above its normal level in Budapest on Sunday.

This means there is a high likelihood that densely populated riverside suburbs in northern Budapest will be affected by flooding. (dpa)