RIYADH, Nov 24 (Reuters) - Representatives from Russia and Ukraine met in the United Arab Emirates last week to discuss the possibility of a prisoner-of-war swap that would be linked to a resumption of Russian ammonia exports, which go to Asia and Africa, via a Ukrainian pipeline, three sources with knowledge of the meeting said.The sources said the talks were being mediated by the Gulf Arab state and did not include the United Nations despite the U.N.'s central role in negotiating the ongoing initiative to export agricultural products from three Ukrainian Black Sea ports.However the talks aim to remove remaining obstacles in the initiative extended last week and ease global food shortages by unblocking Ukrainian and Russian exports, they added.Ukraine's President Volodymyr Zelenskiy has publicly set several conditions before allowing Russia to resume its ammonia exports via the pipeline, including a prisoner swap and reopening of Mykolaiv port in the Black Sea.Since July, Moscow has repeatedly said its shipments of grain and fertilisers, though not directly targeted by sanctions, are constrained because sanctions make it harder for exporters to process payments or to obtain vessels and insurance."