An exchange of gunfire rocked Yemen' s capital Sanaa Wednesday between opposition tribal leader Sadiq al-Ahmar's followers and forces of President Ali Abdullah Saleh, hours after the veteran ruler arrived in Riyadh to sign a power transfer deal, witnesses told Xinhua.

Shootings and a series of huge explosions could be heard in the Hassaba district in central Sanaa, several witnesses told Xinhua.

No casualties have been reported.

Sadiq al-Ahmar, chieftain of the powerful northern Hashid tribal confederation, joined the protests demanding Saleh's ouster earlier this year.

Hassaba district was the scene of deadly clashes in May between the rival forces, which left over 300 people killed.

The fresh escalation came hours after Saleh arrived in Saudi Arabia's capital Riyadh to attend the ceremony of signing the UN- backed Gulf Cooperation Council initiative to transfer power peacefully to his deputy, Abd-Rabbu Mansour Hadi, in return for immunity from prosecution.

Saleh has backed down from signing the deal for three times in the last minute since the deal was offered in April.

Vice President Hadi and the opposition delegation are on their way to Riyadh Wednesday to sign the initiative and its implementation mechanism proposed by UN envoy to Yemen Jamal bin Omar.

Yemeni government officials said the signing ceremony will be held Wednesday evening.