At least nine people were killed in heavy fighting in the Yemeni city of Taiz on Friday, witnesses and medical officials said, a day after a U.N. envoy began a new mission to secure the removal of President Ali Abdullah Saleh under a Gulf peace plan.

They said at least 23 people were wounded in the fighting between Saleh's Republican Guards and opposition tribesmen in the centre of Yemen's third largest city, a hotbed of anti-Saleh protests some 200 km (120 miles) south of Sanaa.

Witnesses said two children and a woman were among the nine people killed in tank and mortar fire aimed at the al-Rawda and Zaid al-Moshki districts as well as Freedom Square, where demonstrators demanding an end to Saleh's 33-year-long rule gather for noon prayers every Friday.

The fighting broke out on Thursday after gunmen shot and critically wounded a soldier stationed at a government building. This was followed by the killing of a pro-Saleh tribal leader and the wounding of one of his bodyguards.

U.N. envoy Jamal Benomar arrived in the capital Sanaa on Thursday on what a spokesman said was a mission to encourage an inclusive transition process that meets the needs and aspirations of all Yemenis.

Martin Nesirky, spokesman for U.N. Secretary-General Ban Ki-moon, said Benomar would report back to Ban, who is to inform the U.N. Security Council about the implementation of a resolution adopted last month that called on Saleh to accept a Gulf-brokered plan under which he would step down.

Saleh has repeatedly backed out of signing the agreement.

(Reporting by Khaled Abdullah in Taiz and Mohammed Ghobari in Sanaa; Writing by Sami Aboudi; Editing by Alistair Lyon)