The Hungarians are concerned that the vehicles could be used by the Yugoslavs for military purposes.

The 75-vehicle convoy was stopped when it tried to cross from Ukraine.

Hungarian officials said five of the vehicles had armoured cabins and contravened United Nations resolutions forbidding the export of military goods to Yugoslavia.

There is also a dispute over whether to allow through fuel tankers, which are carrying 56,000 tonnes of diesel.

There was a shortage of diesel in Yugoslavia even before the Nato attacks began, although the Russians say the fuel is needed for the field hospital the convoy is carrying.

Hungary 'helping Nato'

Russia has reacted angrily to the Hungarian action, saying the affair could damage bilateral relations.

Nick Thorpe: "There is now a serious shortage of fuel"

The Russian Emergencies Minister, Sergei Shoygu, has gone to Budapest to try to get clearance for the convoy.

Russian officials suggest Hungary is deliberately obstucting the convoy out of loyalty to Nato, which it recently joined.

Earlier objections to the use of some armoured vehicles carrying the aid were resolved when the cargoes were transferred to other trucks.

Food, medicine and blankets

The trucks are carrying humanitarian aid for Serbia consisting of a field hospital, medical supplies, food and blankets.

Organised by the Russian Ministry for Emergency Relief, it had been due to arrive in Belgrade on Sunday morning as a good-will gesture from fellow orthodox Christians in time for the Christian Orthodox Easter.

Even before the Nato bombing campaign started, there were some 700,000 Serbian refugees in Yugoslavia, mostly from the Krajina region of Croatia and from Bosnia.