A civilian was the latest Yemeni to be killed in shelling in Taez where clashes have cost the lives of more than 30 people since last week, medics said on Tuesday, as the United Nations sounded the alarm.
In Geneva, UNICEF said that three children, including a three-month-old baby, have been killed in the latest eruption of violence in the country's second largest city.
UN human rights chief Navi Pillay said it was "appalling" that government forces "continue to use live ammunition against unarmed protesters."
Medics said that an artillery round fired by troops loyal to President Ali Abdullah Saleh hit a bus heading towards the restive northern Al-Hasab district of the flashpoint city late on Monday.
"One passenger was killed while several others were wounded," a medic said.
After a lull on Sunday, renewed clashes between armed tribesmen and Saleh's forces erupted in Taez late on Monday.
Two women were killed and six people were wounded when Saleh's forces fired on a crowd of anti-regime protesters, also on Monday.
The latest deaths brought to 34 the number of people killed in Taez since Thursday.
Brief overnight clashes between tribesmen loyal to dissident tribal chief Sheikh Sadiq al-Ahmar and Saleh's troops also shook the northern Al-Hasaba district of the capital Sanaa, but no casualties were reported.
UNICEF sounded the alarm over the increase in violence in Yemen, saying children were paying a price.
"Our numbers are showing that three children have been killed and seven injured in this latest round of violence in Taez," spokeswoman Marixie Mercado told journalists.
"That brings the total number of children killed so far to 138, the majority through live ammunition," since the uprising began in late January, she said.
The agency estimates that a further 568 children have been injured.
"Our youngest victim thus far was three months old, killed in Taez on December 1," Mercado said.
The Yemen Humanitarian County Team, comprising UN agencies and local and national relief organisations, said access to basic services was increasingly limited and schools and hospitals had been occupied or come under attack.
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UNICEF called for immediate access into Yemen so its staff can assess the situation.
UN High Commissioner for Human Rights Pillay condemned the new surge in violence.
"Unarmed civilians, including very young children, have been shot and left dead or with serious injuries," she said in a statement.
"It is appalling and extremely disappointing that despite the successive deals and ceasefires, government security forces continue to use live ammunition against unarmed protesters."
Hundreds of people have been killed nationwide since the uprising began.
On November 23, Saleh signed a Gulf-brokered and UN-backed transfer deal to transfer power to his deputy, Abdrabuh Mansur Hadi, in return for immunity from prosecution.
Opposition spokesman Mohammed Qahtan accused "Saleh and his sons" of orchestrating the violence in Taez.
Saleh's son Ahmed commands the elite Republican Guards, who have been repeatedly locked in deadly confrontations with dissidents.
On Sunday, Hadi formed a military commission under the Gulf Cooperation Council agreement to oversee the restructuring of the security forces, many of which are controlled by Saleh's relatives.
The official Saba news agency said the 14-member commission would also oversee the withdrawal of gunmen from the streets.
Meanwhile, Saba said on Tuesday that 110 officers and soldiers from the First Armoured Brigade led by dissident General Ali Mohsen al-Ahmar had been arrested.
Led by General Sadiq Ali Sarhan, the soldiers were sent to Taez to "implement a terrorist plan by (the Islamist) Al-Islah (reform) party and its civilians and army branches and arms... to occupy the city and cause trouble," it said.
The statement accused General Ahmar, whose troops protect protesters in Sanaa, of plotting to seize control of "Yemeni cities one after the other and carry out assassinations" in Taez.
The developments came as prime minister-designate Mohammed Basindawa was expected to announce a national unity government within two days, a European diplomat and a Yemeni official said on Sunday.
"There are no problems regarding the government. It will be formed today or tomorrow," Qahtan said on Tuesday.
Under the Gulf initiative, half of the government must be opposition members, with regime loyalists making up the other half.