HARARE, Zimbabwe (Reuters) -- President Robert Mugabe's guards briefly detained the U.S. ambassador to Zimbabwe after he entered a restricted security zone near the African leader's residence, state television reported Thursday.The station said U.S. envoy Christopher Dell was held Monday by the Presidential Guard after he entered the restricted zone at the National Botanic Gardens near Mugabe's official Harare residence, ignoring "no entry" signs.A U.S. State Department official said the department was looking into reports of an incident involving the ambassador and was trying to ascertain exact details before commenting further.Zimbabwe's government and U.S. embassy officials in Harare could not be reached for comment late Thursday.State television said Zimbabwe's Foreign Affairs Ministry had written to the United States over what it called "a calculated disregard of the rules governing relations between states ... clearly intended to provoke an unwarranted diplomatic incident."It quoted a defense forces spokesman as saying individuals who entered restricted zones risked being shot and "the American diplomat had the men on duty to thank for their tolerance and restraint in executing their duty."Relations between the United States and Zimbabwe have soured in recent years, with Washington accusing Mugabe's government of rigging parliamentary and presidential elections since 2000 and human rights abuses.Last month a senior U.S. official said President Bush's administration planned tough sanctions barring Mugabe, members of his government and their extended families from traveling to the United States.Mugabe, in power since independence from Britain in 1980, says the United States, like several other Western countries, is bitter over his forcible redistribution of white-owned farms among blacks and has helped sabotage Zimbabwe's economy in retaliation.Copyright 2005 Reuters. All rights reserved.This material may not be published, broadcast, rewritten, or redistributed.