Heavy fighting has erupted in the Yemeni capital, Sana'a, between forces loyal to President Ali Abdullah Saleh and armed tribesman who side with anti-Saleh protesters.

Witnesses said explosions and heavy gunfire broke out early Thursday in the north of the city, with government troops battling followers of tribal leader Sheikh Sadek al-Ahmar.

There was no immediate report of casualties.

Demonstrations against Yemen's embattled president have escalated during the past two weeks, with clashes between the rival sides leaving more than 100 people dead.

Yemen's foreign minister has blamed the turmoil on the opposition movement's refusal to accept 2006 presidential results. Abu Bakr al-Qirbi said Tuesday that Mr. Saleh is committed to pushing forward a long-stalled plan to transfer power to a deputy.

Mr. Saleh has agreed to the proposal three times since April, but has backed out each time before it could be signed.

He returned to Sana'a on Friday after a three-month stay in neighboring Saudi Arabia to recuperate from a June assassination attempt.