These difficulties could become perilous, as there is a realisation that the troops are close to crossing what Lieutenant-General Sir Michael Rose, the former UN commander in Bosnia, calls "the Mogadishu line" between peace-keeping and fighting a war.
The problems include a lack of momentum in negotiations with Serbia, an absence of clarity over the new UN mandate for the troops and the Bosnian government's complaint that Western leaders are worried more by the plight of UN hostages than the lives of Bosnian civilians.
President Slobodan Milosevic of Serbia has hardened his negotiating position on the recognition of Bosnia, and is seeking greater concessions. The talks are now delayed.
Britain and France had pinned their hopes on the success of US mediation with Mr Milosevic. Now that they have been denied the impression of diplomatic progress to accompany the military escalation, they may have to grit their teeth and go ahead with troop reinforcements anyway. As they do so, the mission entrusted to the soldiers is becoming fraught with political constraints.
These complexities were made apparent yesterday when the Russian Foreign Minister, Andrei Kozyrev, came to London to press Moscow's view that the peace-keepers should remain absolutely impartial between Muslims, Croats and Serbs.
Mr Kozyrev said he was reassured to learn that the peace-keeping mandate would not substantially change. The Foreign Secretary, Douglas Hurd, told him that Britain would continue to consult Russia, which has traditional links with the Serbs. But British officials made it clear there will have to be changes to the UN mandate to formalise the increase in troop numbers. Mr Koyzrev, in his usual urbane style, has suggested Russia will restrict the margin for manoeuvre when the mandate comes before the Security Council. He warned the West against "new offensives and a new escalation of hostilities".
Britain and France are now talking in terms of the need for a "minimum level of consent" from the warring parties to permit the UN to stay in Bosnia. But officials in London are reluctant to define that level.
Military men on the ground insist on the need to provide convoys with tough escorts to face down challenges. The first such exercise may illustrate what "minimum consent" might mean. A violent clash with one faction would very likely trigger a more robust UN response than in the past, raising the risks of combat between peace-keepers and local fighters.
Mr Hurd will chew over these unpalatable possibilities when he dines tonight with the new French Foreign Minister, Herve de Charette. The two men will probably agree on the nomination of the former Swedish prime minister, Carl Bildt, to succeed Lord Owen as the European Union's mediator in the former Yugoslavia.
That Sisyphean task is likely to become even less rewarding as the Bosnian government concludes that European nations are getting ready to abandon it.
Yesterday, the new Bosnian Foreign Minister, Muhammad Sacirbey, made a sceptical assessment of the UN mission.
Speaking in London, he said he welcomed the preparation of a 10,000- strong reserve force. But Mr Sacirbey said that the force might end up simply "solely focused on protecting itself''.
He added: "We do take exception to the priority that is being placed on the lives of our citizens, which seems to be lower as compared to the lives of the UN personnel, including the UN hostages. The UN mission is becoming more tenuous. There is a real possibility the UN mission in Bosnia could end."
The Bosnian Foreign Minister said his government would co-operate with a UN withdrawal, but warned that civilians might try to stop the troops pulling out.Reuse content