The UN envoy for Yemen expressed concerns on Wednesday about violations of a ceasefire in the war-wrecked country, urging the warring sides to uphold the first nationwide truce in six years. Hans Grundberg said that while the truce has led to a “significant reduction of violence” in Yemen, there were reports of “some hostile military activities,” particularly around the central city of Marib. He did not say which side was to blame for the violations but Yemen’s internationally recognized government has accused theIran-backed rebel Houthis of attacking their positions in southern and western Ma…