Eighteen militants and four Yemeni soldiers were killed in clashes involving the army backed by the air force Thursday, a local official and medics said, near a southern provincial capital seized by Islamist dissidents in May.

The fighting took place in the suburbs of Zinjibar, in Abyan province where the militants have seized several towns, while 11 months of protests against President Ali Abdullah Saleh have pushed Yemen to the brink of civil war.

Heavy fighting has continued all day and the air force and artillery have been used, a local official told Reuters. He said the total death toll had risen to 22.

A medic earlier said the bodies of the dead soldiers and those of some the militants had been moved to a military hospital in the southern port city of Aden.

The government says the militants are linked to al Qaeda's Yemen-based regional wing, which the United States has called the most dangerous branch of the militant network.

Saleh's opponents have accused him of ceding territory to Islamists to bolster his assertion that his rule keeps al Qaeda in check.

Neighbouring oil giant Saudi Arabia and Washington fear that continued unrest in Yemen may embolden its al Qaeda wing, which has claimed responsibility for operations that include a failed plot to blow up a U.S.-bound passenger plane in 2009.

In Sanaa, Hamid al-Ahmar -- a tribal leader, wealthy businessman and a member of the Islamist opposition party Islah -- called for Saleh to be put on trial, opposing any move to grant him immunity under a transition deal aiming to end nearly a year of unrest in the poor Arabian Peninsula country.

The government's proposed immunity for Saleh is tantamount to neglecting the blood of the Yemeni people, Ahmar said in a statement, referring to the hundreds killed during the protests.

Ahmar called for Saleh to be charged, echoing a demand raised by demonstrators across Yemen since the transition deal sponsored by the Gulf Arab states was signed in November to end Saleh's 33-year rule.

Separately, Yemen's transport minister told Reuters on Thursday that Sanaa would renegotiate a joint venture agreement with DP World to run the Aden container port, accusing the Dubai-based operator of failing to fulfill its obligations.

(Reporting by Mohammed Mukhashaf in Aden and Mohammed Ghobari in Sanaa; Writing by Firouz Sedarat; Editing by Philippa Fletcher)