An AFP journalist reported gunshots and tear gas were fired by security personnel as they faced off with several thousand demonstrators outside the regional government headquarters.

The measures failed to disperse protesters, who responded by hurling Molotov cocktails and letting off fireworks at the security forces.

Basra and the surrounding province have been the focus of angry demonstrations over government neglect that have rocked Iraq since early July.

Residents are particularly angry over pollution of the local water supply, which has put 20,000 people in hospital.

On Tuesday six demonstrators were killed and more than 20 wounded during the bloodiest day of clashes with security officials, a local official and medics said.

The authorities said that 30 security personnel were wounded in the violence.

The United Nations envoy to Iraq has called for "calm" in Basra and urged the authorities "to avoid using disproportionate, lethal force against the demonstrators".

In his weekly press conference in Baghdad on Tuesday, Iraqi Prime Minister Haider al-Abadi said he had ordered "no real bullets ... to be fired, in the direction of protesters or in the air".

Shiite leader Moqtada Sadr said in a tweet ahead of the latest clashes that "vandals infiltrated" the protesters.

Sadr's political bloc won the largest number of seats in national elections held in May, and he is trying to form a new government with Abadi.

In July, the government announced a multi-billion dollar (euro) emergency plan for southern Iraq, to revive infrastructure and services.

But protesters are wary of promises made by the outgoing government, as negotiations drag on over the formation of the next administration.