Geneva - The United Nations urged authorities in Houthi-controlled Sanaa on Thursday to investigate a "grave attack" on the convoy of its envoy to Yemen during his visit to the capital. U.N. special envoy, Ismail Ould Cheikh Ahmed was in Sanaa for three days of talks aimed at preventing military action at the strategic port of Hodeidah, the entry point for 70 percent of Yemen's food supplies as well as humanitarian aid. A U.N. statement said that Ahmed's convoy came under attack while traveling from the airport in Sanaa to the United Nations compound. The Houthi-run Saba news agency has denied any attack. The U.N. statement gave no details, but local officials said shots were fired toward the convoy by unknown assailants. "The Special Envoy expressed his deep concern regarding the grave attack on his convoy while traveling from the airport to the UN compound on 22 May," the U.N. statement said. "The Special Envoy reminded the parties that it is the responsibility of the local authorities to ensure the safety of all UN personnel in the country and urged them to investigate the incident, hold those responsible to account, and prevent any such incidents in the future," it added.