Serbian police turned back a U.N. aid convoy carrying supplies for 30,000 refugees from the Kosovo conflict on Thursday as a senior U.N. envoy began an inspection of humanitarian conditions.

Fernando Del Mundo of the U.N. High Commissioner for Refugees said the eight-truck convoy was stopped by police at Slatina near Pristina and told to return to base.The convoy had been trying to reach Pec in western Kosovo with provisions for people made homeless by six months of fighting between Serbian security forces and ethnic Albanian separatists of the Kosovo Liberation Army.

UNHCR headquarters in Geneva said it was protesting against Serb action, but police told reporters that sniper fire along the road made it too dangerous for the convoy to make the journey

U.S. assistant Secretary of State Julia Taft headed into western Kosovo to see the plight of an estimated 200,000 refugees and displaced persons.

Taft was accompanied by Richard Miles, the U.S. charge d'affaires in Yugoslavia.

A gas bomb was hurled at the entrance to the U.S. Information Center in Pristina on Wednesday evening, but diplomats said it was not clear whether the incident was connected with her visit.