28 people have been killed in an explosion at a weapons storage facility in Yemen's capital Sana'a, according to the defence ministry.
The opposition, however, disputed the government's account and said the latest deaths occurred early on Thursday during heavy shelling of a residential area by troops loyal to Yemen's embattled President Ali Abdullah Saleh.
Government troops have clashed with fighters led by Yemen's most powerful tribe for four straight days. The latest deaths bring the toll to at least 109 killed.
The battles broke out Monday after Mr Saleh's troops tried to storm the compound of Sheik Sadeq al-Ahmar, the leader of the largest tribe, called the Hashid.
Al-Ahmar's forces have laid siege to at least nine government ministries and government troops have responded with shelling on al-Ahmar's compound in the heart of Yemen's capital.
On Thursday, Yemeni authorities escalated its campaign against al-Ahmar by issuing an arrest warrant against him and other tribal leaders.
The US State Department has ordered non-essential diplomats to depart Yemen and urged all Americans there to leave as security conditions deteriorated with the country's embattled leader refusing to step down.
The decision to tell most non-essential personnel and the families of all American staff at the US Embassy in Sanaa to leave was a sign of Washington's increasing concern about the situation in Yemen, where street battles between supporters and opponents of Mr Saleh raged for a third day. The clashes have left at least 41 dead and dozens badly injured.
The "ordered departure" notice came in a new travel warning for Yemen released as the Obama administration stepped up calls for Mr Saleh to transfer power under an agreement negotiated by neighbouring Persian Gulf states.
Speaking in London, President Barack Obama called on Mr Saleh to "move immediately" to implement the agreement.
An earlier US travel alert for Yemen issued in March had allowed non-essential embassy staff and their families to leave at government expense. It had also urged Americans not to go to Yemen but had only told those already in the country to consider leaving.
The new alert followed a defiant message from Mr Saleh, who vowed not to step down or allow Yemen to become a "failed state." His stance, combined with renewed fighting, sharply increased chances that Yemen's three-month uprising could turn into a militia-led revolt after Arab mediation failed to crack his 32-year authoritarian rule.
"I will not leave power and I will not leave Yemen," a spokesman, Ahmed al-Soufi, quoted Mr Saleh as saying.
He also took a direct swipe at US-backed efforts to negotiate his exit. "I don't take orders from outside," said the statement, read by the spokesman in a meeting with tribal allies.
"Yemen will not be a failed state. It will not turn into an al-Qaeda refuge."