ZAGREB, May 2 (Reuter) - The United Nations said Croatian troops entered the Serb-held town of Okucani, in the centre of their western Slavonian enclave, dealing a serious blow to rebel Krajina Serb forces. "We received a report from the Nepalese (U.N. soldiers) just after two o'clock (1200 gmt) that Croatian troops and tanks entered Okucani," said U.N. spokesman Susan Manuel by telephone on Tuesday from the Western Slavonia region.