Renewed clashes have broken out in the Yemeni capital Sanaa between government troops and forces opposing President Ali Abdullah Saleh, witnesses say.
A medic told AFP news agency that snipers had shot dead two women in a protest camp dubbed Change Square in the city centre.
Two men were also killed by mortar fire in the same area and at least nine other people were injured, reports say.
More than 80 people, mostly unarmed protesters, have died since Sunday.
Witnesses say rival forces are exchanging gun and shell fire in the north of the city and the battles are spreading across neighbourhoods.
Laura Kasinof, a correspondent for the New York Times in Sanaa, tweeted that artillery was firing from nearby mountains.
A resident told AFP that schools, banks and businesses in the city were shut.
“The city is empty… the ghost of war looms over Sanaa,” the resident said.
The BBC has not been able to independently verify the reports.
Correspondents say time for a diplomatic solution is fast running out as Yemen appears to be sliding into full-blown civil war.
On Tuesday the government agreed a truce following negotiations with Western envoys.
However, the ceasefire broke down hours later and at least three people were reportedly killed by sniper fire and shelling on Wednesday.
A fresh attempt by the Gulf Co-operation Council (GCC) to mediate a solution to the crisis ended on Wednesday without success.
Government forces are battling army elements who have defected to the opposition as well as armed tribal groups.
Activists, inspired by the Arab Spring, have been camped out for eight months in Change Square demanding an end to President Saleh’s 33-year rule.
Mr Saleh is recovering in Saudi Arabia after being wounded in an assassination attempt in June.
He is refusing to transfer power and has vowed to return to Yemen.