Armed Yemenis loyal to tribal chief Sheikh Sadiq al-Ahmar, who heads a coalition backing anti-regime protests, were deployed in Sanaa on Friday as army troops bore down on their stronghold in the capital.
Armoured vehicles from the elite Republican Guard, led by President Ali Abdullah Saleh's son Ahmed, advanced overnight towards the northern Al-Hassaba neighbourhood as tribesmen dug trenches in anticipation of renewed clashes, an AFP correspondent reported.
The troops blocked several roads and erected checkpoints, triggering a similar action by the tribesmen armed with machine guns and rocket-propelled grenades.
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Tensions were high despite mediation attempts, the correspondent said, pointing out that loyal troops got very close to the tribe's stronghold and the First Armoured Division, which has sided with anti-Saleh protesters.
Fierce clashes between Ahmar's tribesmen and loyal troops in Al-Hassaba in May killed more than 140 people before the two sides agreed a truce.
The sheikh heads the powerful Hashid tribe which ended its support to Saleh -- also a member of Hashid -- in March and joined the protests which erupted in January demanding the ouster of the president who has ruled since 1978.
Influential tribal leaders formed last week a coalition headed by Ahmar to bolster the uprising against Saleh who has been in a Saudi hospital since June after being wounded in a bomb attack on his Sanaa compound.
Tribes wield much influence in impoverished Yemen, where the Hashid is a heavily armed tribal confederation capable of rallying and financing thousands of fighters. The Bakil is the other main tribal confederation.