Thirty-seven people, most of them combatants, were killed in overnight clashes in Sanaa, Yemen, between opposition tribesmen and security forces, a medic at Jumhuriya Hospital said on Wednesday.

There was heavy fighting in the capital throughout the night, an AFP correspondent said.

Clashes between tribesmen loyal to Sheikh Sadiq Al-Ahmar, who heads the powerful Hashid confederation, and security forces broke out in Sanaa on Tuesday shattering a truce announced on Friday.

The government of veteran President Ali Abdullah Saleh accused Ahmar's fighters of breaking the truce.

The defence ministry's 26sep.net news website said that his tribesmen had seized both the headquarters of the ruling General People's Congress and the main offices of the water utility.

But sources close to Ahmar said Saleh's forces were to blame as they had opened fire on the tribal leader's compound in the north of the capital.

Ahmar announced on Friday that "there is a truce between us and Ali Abdullah Saleh" for mediation to take place, speaking at a funeral for 30 of his fighters killed in four days of clashes with Saleh's forces in the capital last week. But he added: "If the Saleh regime wants a peaceful revolution, we are ready for that. If he chooses war, we will fight him."

In March, Ahmar pledged his support for protesters who have been demonstrating since January for the departure from office of Saleh, who has been in power since 1978.