More than 60 people have now been confirmed killed in the fighting since a fragile truce between the forces of powerful tribal leader Sheikh Sadiq al-Ahmar and embattled President Ali Abdullah Saleh collapsed on Tuesday.
But medics said they had no word yet on casualties from Thursday's fighting as ambulance crews were unable to access the Al-Hasaba neighbourhood of north Sanaa where Ahmar has his base.
"The bodies are still scattered in Al-Hasaba and ambulances cannot reach it due to the dangerous situation there," a medical official said.
|Yemeni anti-government protesters shout slogans during a demonstration calling for the ouster of President Ali Abdullah Saleh in Sanaa amid heavy fighting in the city|
At least 15 more people died in overnight fighting, medics said, adding to 47 killed on Tuesday and Wednesday.
Residents said the fighting was the fiercest so far after loyalist special forces who had received US training as part of Washington's "war on terror" joined the battle.
Thousands of armed tribesmen were on their way to Sanaa to boost Ahmar's forces, tribal leaders said.
An advanced guard clashed with loyalist troops when it was stopped at a military post 15 kilometres (nine miles) north of the capital, they added.
Witnesses said a warplane broke the sound barrier over Ahmar's hometown of Khamr, 80 kilometres north of Sanaa, in an apparent attempt to intimidate the tribesmen of his Hashid confederation.
Mohsen Sinan, 70, said he and 30 members of his household were trying to flee Sanaa along with many other residents. "Sanaa is deserted now and if these battles continue, Yemen will be finished," he said.
Many shops were closed and long queues formed outside petrol stations. Those residents who remained in the city complained of water shortages and power cuts.
Saleh, who has been in power in Sanaa since 1978, has faced nationwide protests against his rule since mid-January.
Ahmar, who heads one of the impoverished Arabian Peninsula nation's two main tribal confederations, threw his weight behind the protesters in March.
When Saleh last month refused to sign a plan brokered by Yemen's wealthy Gulf Arab neighbours for him to step down in return for a promise of immunity from prosecution, Ahmar's fighters seized a string of public buildings across the capital sparking clashes with troops loyal to the president.
A truce announced last week lasted just four days before clashes resumed with each side blaming the other.
A government spokesman on Thursday raised the possibility that Saleh might finally give in to international pressure and sign up to the transition plan drawn up by the six-nation Gulf Cooperation Council.
"The date for the signing will be set soon based on consultations and coordination between Yemen and the Gulf Cooperation Council states," the official Saba news agency quoted the spokesman as saying.
The Senegalese president's office said late Thursday that Saleh had asked President Abdoulaye Wade to approach France and the United States in a bid to get a ceasefire followed by elections in Yemen in which Saleh would not stand.
"President Saleh requested President Wade's intervention with France, the United States, Saudi Arabia, the United Arab Emirates and other countries to create the conditions for an immediate ceasefire and the programming of free and transparent elections whose results he pledges to accept," the office said in a statement received by AFP.
Saleh "said he did not intend to stand in these elections", said the statement issued after the embattled Yemeni leader held a telephone conversation with Wade, who is chairman of the Organisation of the Islamic Conference.
Senegal indicated that it was prepared to accept the Yemeni leader but Saleh retorted that he "preferred to stay in his country".
In Yemen's second city Taez, protesters again took to the streets on Thursday, for the first time bearing weapons, witnesses said.
The protesters clashed with troops of the Republican Guard, an elite unit commanded by Saleh's son Ahmed, the witnesses said, adding that there were no immediate reports of casualties.
Loyalist troops smashed a long-running sit-in in the centre of Taez earlier this week, leaving more than 50 protesters dead, according to the UN human rights office.
Nationwide, well over 200 demonstrators have died since the protests first erupted in January, according to an AFP tally based on reports from medics.
In the south, two soldiers were killed and 15 wounded in fighting with militants who have seized control of much of the city of Zinjibar, a doctor at the Nasahib military hospital in the regional capital Aden said