At least 11 people have been killed in heavy fighting in the Yemeni city of Taiz, a day after a UN envoy began a new mission to push President Ali Abdullah Saleh to resign under a Gulf-backed peace plan.

Witnesses and medical staff said at least 25 people were also wounded on Friday in fighting between Saleh's Republican Guards and opposition tribesmen in Yemen's third largest city, a centre of anti-Saleh protests some 200km south of the capital Sanaa.

Abdulrahim al-Samie, an anti-government activist in Taiz, told Al Jazeera that "what is happening in Taiz today is a real war against civilians".

Witnesses said two children and three women were among those 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 had gathered for noon prayers.

The fighting in Taiz started 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.

UN pressure

UN envoy Jamal Benomar arrived in Sanaa, the capital, on Thursday to encourage "an inclusive transition process that meets the needs and aspirations of all Yemenis", said Martin Nesirky, spokesman for UN Secretary-General Ban Ki-moon.

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Nesirky said that Benomar would report back to Ban, who is to inform the UN Security Council about the implementation of a resolution adopted last month that called on Saleh to accept a Gulf Co-operation Council-brokered plan under which he would step down.

Saleh, who has clung to office despite pressure at home and abroad, has repeatedly failed to signing the deal - which calls for him to hand power to his deputy, Abd-Rabbu Mansour Hadi - despite promising to do so.

Under the deal, Hadi would oversee the formation of a national unity government ahead of an early presidential election.

France has said that the European Union will discuss freezing Saleh's assets to increase the pressure for his departure.

International governments are fearful that violence in Yemen may strengthen al-Qaeda fighters based there who have launched attacks in the past on US and Saudi targets.