DUBAI (Reuters) - 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.

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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.

The General People’s Congress, the party of former President Ali Abdullah Saleh, who is allied with the Iranian-aligned Houthis, condemned the attack and also called for an investigation.

However, the Houthi-run Saba news agency has denied any attack on the convoy and said gunshots were fired into the air by security forces assigned to guard the envoy’s mission to disperse demonstrators trying to block his path.

The Saudi-led coalition that has been fighting since March 2015 to end Houthi rule and restore President Abd-Rabbu Mansour Hadi to power have demanded that the port of Hodeidah be handed over to international control to spare it an attack.

The coalition has accused the Houthis of using the port to smuggle in weapons and ammunition, which the Houthi’s deny.

The frontline of the civil war that has killed more than 10,000 people is in Midi, 230 km (140 miles) north of Hodeidah near the border with Saudi Arabia, and in the south outside al-Khoukha region, 130 km (80 miles) south of Hodeidah.

According to the U.N. statement, Ould Cheikh Ahmed also discussed ways to ensure the resumption of salaries to Yemeni civil servants who complain that salaries have not been paid on time since Hadi ordered the central bank moved from Sanaa last year to the southern port city of Aden.