Anti-government protesters have demanded the removal of President Ali Abdullah Saleh [AFP]

Two civilians died on Thursday when a shell hit their house in the al-Hassaba district of the capital, Sanaa, a source in the office of Sheikh Sadiq al-Ahmar, a tribal chief, said.

Mohammad al-Qadhi, a former managing editor of Yemen Times, told Al Jazeera that clashes had taken place between fighters supporting Sadiq, the leader of the most powerful tribal federation, and troops loyal to President Ali Abdullah Saleh.

Al-Ahmar is among the tribal leaders calling for the departure of President Ali Abdullah Saleh. Two of his fighters were wounded in the shelling, the source said.

Elsewhere in Yemen,  a 14-year-old boy was killed on Thursday and five other people were wounded during an exchange of fire that followed two blasts near a police station in the port city of Aden.

A local security official said the blasts were probably caused by explosive devices that did not manage to penetrate the perimeter walls of the two buildings, which stood around 400 metres apart.

A third explosion targeted an office of the intelligence services, the source said.

Saleh, Yemen's leader of nearly 33 years, has held onto power in the face of massive protests demanding his resignation since February. He is currently recuperating in Saudi Arabia after a June attack on his compound.

UN human rights officials gave warning on Tuesday of an impending civil war unless the political deadlock was resolved.