A tenuous truce declared a few days ago to end street fighting in the Yemeni capital between tribal groups and forces loyal to Ali Abdullah Saleh, the embattled president, has broken down, sending the country closer to the brink of civil war.
"The ceasefire agreement has ended," a government official said on Tuesday without giving further details.
The announcement came as overnight clashes in Sanaa killed many people and left dozens injured.
Sources told Al Jazeera that the heaviest shelling took place near the interior ministry building and the house of Sadiq al-Ahmar, a powerful tribal leader ranged against President Saleh.
They said forces loyal to Saleh, under pressure from protesters to quit and end his 33-year rule, fired tens of shells and missiles from a mountain near the house of al-Ahmar.
A journalist from Sanaa told Al Jazeera the fighting was the fiercest the capital had seen in a long time.
"People are leaving, several homes were burned and tribal forces took over some government buildings and police stations," the journalist said.
Protesters 'shot dead'
Separately, security forces reportedly shot dead at least two anti-government protesters in Yemen's second-largest city of Taiz on Tuesday, witnesses said.
They said that security forces were attempting to prevent anyone from gathering in the city, firing on those who tried to do so.
Medics confirmed that at least two people had been killed.
Tuesday's deaths came after protesters said security forces smashed a four-month-long sit-in in Taiz on Monday, killing 21 protesters.
According to reports received by the UN, more than 50 protesters have been killed in Taiz since Sunday.
"The UN human rights office has received reports, which remain to be fully verified, that more than 50 people have been killed since Sunday in Taiz by Yemeni Army, Republican Guards and other government-affiliated elements," Navi Pillay, the UN human rights chief, said.
The latest violence follows the death of at least 30 people, reportedly killed by air strikes in the southern city of Zinjibar, which is said to be controlled by fighters linked to al-Qaeda.
The air attack on Monday appeared to be in response to Sunday's takeover of the city by 300 alleged al-Qaeda fighters and an overnight ambush that killed at least six Yemeni soldiers and injured dozens more who were travelling to the southern city.
"Civilians found a military car and an armoured vehicle. They were destroyed, and the bodies of six soldiers were found on the roadside," Ayman Mohamed Nasser, editor-in-chief of Attariq, Aden's main opposition paper, told the Reuters news agency by telephone.
Opposition leaders have accused Saleh of deliberately allowing Zinjibar, near a sea lane where about 3 million barrels of oil pass daily, to fall to al-Qaeda in a bid to show how chaotic Yemen would be without him.
Global powers have also been pressing Saleh to sign a deal brokered by Arab Gulf states under their umbrella organisation, the Gulf Co-operation Council, to hand over power.
Under the deal, Saleh was to hand over power in 30 days and be granted immunity from prosecution. The opposition signed the deal but Saleh refused to sign it.
Fears over al-Qaeda
The deal was aimed at stemming the growing chaos in Yemen, home to al-Qaeda fighters and neighbour to the world's biggest oil exporter, Saudi Arabia.
The US and Saudi Arabia, both targets of attacks by Yemen-based al-Qaeda in the Arabian Peninsula, are worried that chaos is emboldening the group.
Saleh has been losing support as protests continued.
A breakaway military group called for other army units to join them in the fight to bring Saleh down.
Under Saleh, Yemen has moved to the brink of financial collapse, with about 40 per cent of the population living on less than $2 a day and a third facing chronic hunger.
At least 320 people have been killed in fighting in Yemen since protests calling for Saleh to end his rule started about four months ago, inspired by the popular uprisings that ended the reign of the long-standing rulers of Tunisia and Egypt.