The foreign secretary, Jack Straw, is to press the Yugoslav president, Vojislav Kostunica, tomorrow about the illegal supply of weapons to west Africa and Iraq.
British officials said yesterday that Mr Straw, who leaves for Belgrade today, would express concern about arms sales from Yugoslavia to Liberia and Iraq in breach of United Nations sanctions.
One of the officials described details uncovered in the last two weeks about the volume of sales to Iraq as "the tip of the iceberg".
Although the focus of media attention is primarily on Iraq, officials said that arms to Liberia were a cause of immense concern because of the threat to regional stability in west Africa. Liberia has been heavily implicated in the civil war in Sierra Leone.
British intelligence, which closely monitors the illegal trade in diamonds and weapons between Sierra Leone and Liberia, claims to have knowledge of two shipments flown to Liberia by an arms dealer from Yugoslavia.
Mr Straw is to seek Mr Kostunica's cooperation in investigating the dealer.
British officials hinted that Yugoslavia's desire for closer links with the European Union and the continued flow of EU aid could be blocked if Mr Kostunica did not comply.
One official said: "Liberia continues to be a threat to the region. Belgrade should be investigating the apparent breach of sanctions by one of its citizens."
A UN report last month alleged that 200 tonnes of weapons had been flown to the Liberian capital, Monrovia, in June and August.
British soldiers intervened to help stop the civil war in Sierra Leone two years ago and 400 of them are still there.
Mr Straw will point out to Mr Kostunica that British troops could find themselves being fired at in Sierra Leone and Iraq by weapons that originated in Yugoslavia.
Belgrade conceded last week that a state arms contractor had violated UN sanctions against Iraq by overhauling Baghdad's military jet engines. Mr Kostunica has so far played this down, saying the contract was only for the "overhaul of old aircraft engines".
Senior Yugoslav officials have highlighted divisions in the government about how to handle the affair, as more evidence of arms deals have emerged.
"I am sure that the military security service... knew it all, to the last document," Yugoslavia's federal interior minister, Zoran Zivkovic, told the Associated Press. "But they failed to warn the government."
The head of the Yugoslav defence conglomerate, Yugoimport, was sacked last month over the contract to equip the Iraqi airforce with a factory to repair and update MiG fighter aircraft.
Yugoslav companies have also been accused by US officials of supplying Iraq with the technology and expertise to develop a cruise missile.
So far the Yugoslav government has dismissed six officials, including a deputy defence minister, in connection with the deal to repair Iraqi jets. Western governments have welcomed this and the announcement of an investigation into the affair, but there is still considerable scepticism about Belgrade's ability and willingness to look into the full extent of the weapons trade.
With substantial sums of money involved, US diplomats following the illegal arms trade say many of the companies and officials involved have little incentive to end their dealings.
In mid September a senior commander of S-For, the international peacekeeping force in Bosnia, warned the Bosnian aviation company, Orao - sub contracted by Yugoimport to build the MiG repairs centre in Iraq - to cease trading.
According to a letter found by S-For troops during a raid on the factory on October 11 and seen by the Guardian, Yugoimport responded to the ultimatum by asking its Iraqi counterparts to cover up any evidence of the deal. It also promised to complete the deal, worth £5.4m.
The letter also proposed new housing for experts working in Iraq to complete the contract, and spoke of taking "apart the equipment over a 10-day period. The Iraqi side will hide them in a safe place. When the possibility of their being discovered passes, the Yugoslav side will reassemble and operate them again".
Most of the recent evidence of breaches of the UN arms embargo involves sales of weapons and spare parts, but evidence has also emerged of Yugoslav academics with missiles expertise visiting Iraq.
Professor Djordje Blagojevic, who teaches missile and aerodynamics, was named by the US embassy in Belgrade as one of several people who had visited Iraq to help improve weapons systems. He denied the accusations in an interview with the Serbian daily, Blic, but admitted he had taught in Iraq last spring.