At least 37 people have died in overnight fighting as clashes intensified in the Yemeni capital Sanaa, doctors and officials say.

Violence escalated after a ceasefire broke down between security forces and fighters loyal to Sheikh Sadiq al-Ahmar, head of a tribal confederation.

Analysts say the conflict threatens to drag Yemen into civil war.

Yemen is facing increasing unrest in several areas after President Ali Abdullah Saleh refused to step down.

Witnesses reported heavy fighting in the capital overnight as both sides blamed each other for breaking the ceasefire.

On Wednesday morning, witnesses also reported loud explosions coming from the north of Sanaa, but the reason was not clear.

The defence ministry accused tribesmen of seizing the headquarters of the ruling General People's Congress and other offices in Sanaa.

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At the scene

Lina SinjabBBC News, Sanaa

People here are extremely concerned that the situation could turn into a civil war, especially if tribes from outside Sanaa join the fighting.

Opposition groups accuse the president of arming civilians and starting the clashes, but they are also angry with tribesmen - especially the Ahmar tribe - for getting involved.

Yemen is a complicated country with a tribal system. Guns are widely available.

In Sanaa, people have started fleeing the area of Hasba where attacks in recent days were concentrated.

Pro-democracy activists are concerned that the violence is detracting from their peaceful demands for change. They are calling on the international community to safeguard their movement.

However, sources close to Sheikh Ahmar said government forces had attacked his compound in the north of the city.

One report said Presidential Guard units had also shelled the headquarters of a brigade where the commander was suspected of being about to defect to the opposition.

Medical officials in Sanaa said casualties included fighters from both sides.

On Tuesday, another 12 protesters were shot dead in the southern city of Taiz following a move by security forces to remove the protest camp from the city centre.

There were also further clashes in the town of Zinjibar where government forces have been battling fighters described as al-Qaeda militants.

Mr Saleh, who has been in office for 33 years, is refusing to sign a deal to transfer power to help end the crisis.

On Tuesday, a US state department spokesman urged him to leave office "and to move Yemen forward"