UN aid chief Stephen O'Brien on Tuesday cancelled a visit to Yemen's third city Taez when shelling and a rebel checkpoint blocked his route, several sources said.
A UN source in Yemen told AFP that O'Brien, who was in a convoy travelling to the conflict zone, had been forced to cancel his visit to Taez "for security reasons".
The convoy that was travelling from Yemen's capital Sanaa had to change course due to shelling on the road leading to the southwestern city, said police captain Oussama Al-Charaabi, the head of government security services in Taez.
A local official told AFP that O'Brien was stopped at a rebel checkpoint in Hizran, 15 kilometres (nine miles) northwest of Taez, while the government-run news agency Saba accused Huthi rebel forces of opening fire at his convoy.
The UN source said O'Brien was now slated to visit a school in the southwestern Ibb province housing internally displaced Yemenis from the Red Sea coastal town of Mokha, controlled by government loyalists since January.
Forces loyal to President Abedrabbo Mansour Hadi, backed by a Saudi-led Arab coalition, have held out in Taez where they are surrounded by the Iran-backed Huthi rebels and their allies.
The rebels control the capital Sanaa and much of the northern highlands of Yemen, where conflict has escalated over the past two years.
O'Brien, who is on a weeklong tour of Yemen, Somalia and Kenya, on Monday warned that seven million Yemenis face "serious risk of famine" unless international donors intervene.
A further 19 million of Yemen's 26 million population now need humanitarian aid, O'Brien said at a news conference.
The United Nations has called for $2.1 billion in humanitarian aid for Yemen, where UN mediation and seven ceasefire accords have failed to end a conflict that O'Brien said has cost more than 7,500 lives and left 40,000 people wounded.
The war in Yemen pits Hadi's internationally recognised government against Huthis and forces loyal to former president Ali Abdullah Saleh.
The fighting has intensified since the coalition intervened in support of Hadi in March 2015 after the Huthis seized Sanaa the previous September.