At least 16 anti-government protestors have been killed and 55 wounded in the latest state crackdown assault on dissent in Sanaa, a medic at a field hospital in Sanaa told reporters.

The focus of protests has been Sanaa's Change Square, where witnesses say troops began attacking the protest camp that has been there since January. The medic, Mohammed al-Qabatis, described soldiers loyal to the regime of President Saleh storming the protesters' encampment in the early hours of Saturday.

A defecting military brigade, headed by General al-Ahmar, has been protecting the protesters, who are calling for an end to decades of President Saleh's rule.

The violence erupted after President Saleh returned from Saudi Arabia on Friday, where he was receiving medical treatment for three months, having survived a shelling of his presidential compound. Saleh called for an end to the violence, which has already claimed about 100 lives this week. Many fear that his return raises the risk of all-out civil war.

US officials urged Saleh to offer the Yemeni people elections. A White House spokesman said:

"The Yemeni people have suffered enough and deserve a path towards a better future."