A threat by the captain of one of the tugs to blow up his convoy of barges and spill tens of thousands of tons of oil into the river was enough to keep the Romanian authorities from using force to stop the convoy - much to the embarrassment of the Security Council in New York, which ordered the sanctions to be put into effect last November.
If a tugboat could pull a fleet of barges up the river from the Black Sea under the eyes of the international community without being interfered with, it made a mockery of the sanctions, diplomats said, and would encourage Serbia to flout the UN's authority further.
In Iraq, by contrast, everything from children's teddy bears to industrial equipment is subject to rigorous international control, with the United States ensuring that very tight sanctions remain in place until such time as Saddam Hussein co-operates with plans to hand all the country's oil export profits to the UN.
The Yugoslav tugboat, Bihac, which made port on Tuesday with a 6,000-ton cargo of Ukrainian diesel oil, is the first ship acknowledged by the UN to have beaten the sanctions on the Danube. Numerous other oil cargoes are believed to have made it up the Danube to Serbia using false papers to get past the often compliant neighbouring border controls.
The Bihac, released from Romanian custody last week after its captain promised to head towards Ukraine, immediately turned upstream and ignored requests to stop from Romanian and Bulgarian customs officers and coastguards. Moreover, the captain threatened to blow up his cargo of oil if force were used to try to halt the tugboat.
After coming under intense diplomatic pressure from the US and the UN, Romania belatedly issued a 'stop-or-else' order to the remaining Yugoslav tugboats making their way up the Danube yesterday and warned that it would use all necessary means - including 'constraint' - to stop their passage.
But, as a Bulgarian official pointed out, its more powerful neighbour may retaliate if force is used. 'Who will guarantee that if we use force to stop the barges the Serbs will not retaliate by attacking our ships in the Danube or Bulgarians transiting via Serbia?' the officials wondered.
All the countries that depend on the Danube as a trading lifeline are vulnerable to Serbian retaliation. Since December, six ships bound for Romania and loaded with diesel oil, sugar, iron and timber, have been detained in the Yugoslav ports of Belgrade, Novi Sad and Bezdan to put pressure on Bucharest to turn a blind eye to the passage of vital oil supplies along the Dabnube.
The sanctions against Serbia are monitored by the Conference on Security and Co-operation in Europe and run by the European Commission's SAM (sanctions assistance missions unit), which helps border guards in countries surrounding Serbia catch sanctions-busters.
The unit has 35 European, US and Canadian customs specialists who work with officers in Hungary, Romania, Bulgaria and Macedonia to try to staunch the flow of goods across Serbia's porous borders.
The SAM has had to contend with a steady stream of Serbian exports leaving the republic without labels or other identification, accompanied by false papers stating Bosnia or Macedonia as the country of origin.
Since September, the SAM unit has caught more than 850 sanctions-busters - but it is operating at full stretch. The 35 foreign customs specialists are trying to plug a border that is 1,000km (625 miles) long and has been notorious throughout the ages for smuggling. Besides trying to watch the busy barge traffic on the Danube, the SAM monitors check on more than 700 trucks that cross the Serbo-Hungarian border every day.Reuse content