Shiite rebels clashed with Sunni Islamists backed by troops north of Sanaa on Thursday, as the UN envoy huddled in the rebel stronghold to try to end Yemen's political crisis.

Fighting raged between the rebels, known also as Huthis or Ansarullah, and fighters of the Islah (Reform) party in Shamlan. Witnesses said five people were wounded.

The fighting has spread to Iman University of religion which belongs to top Islah figure cleric Abdulmajid al-Zandani, they said.

UN envoy Jamal Benomar flew unexpectedly on Wednesday to rebel bastion Saada where he had a three-hour meeting with their leader Abdulmalik al-Huthi.

Benomar called the talks "constructive and positive", and the two are expected to meet again on Thursday.

However, sources close to the talks said they still face hurdles, mainly on the dismantling of protest camps set up by the rebels weeks ago in and around Sanaa.

The rebels earlier rejected an offer by President Abdrabuh Mansur Hadi to name a new premier and reduce a controversial fuel price rise, two core rebel demands.

The Shiite Zaidi community, to which the Huthi rebels belong, is a minority in mainly Sunni Yemen but a majority in the northern highlands, including in the Sanaa region.

Analysts say they are trying to establish themselves as the main political force in those regions.