The UN said it cannot keep track of the death toll in Syria's months-long unrest that has already cost more than 5,400 lives, as government forces targeted the protest hub of Hama with a major assault.
The admission came as European and Arab nations worked on a UN Security Council resolution condemning President Bashar al-Assad's government for its continuing deadly crackdown on pro-democracy protesters.
UN human rights chief Navi Pillay gave a toll of more than 5,000 dead when she spoke to the UN Security Council in early December, but has not updated it.
Under secretary general B. Lynn Pascoe told the council on 10 January that at least 400 people had been killed since Arab League monitors arrived in Syria on 26 December.
After meeting Security Council ambassadors again, Pillay said the toll had risen but added: "We are experiencing difficulties because of the fragmentation on the ground.
"Some areas are totally closed such as parts of Homs, so we are unable to update that figure but in my view 5,000 and more is a huge figure and should really shock the international community into taking action," she told reporters.
Russia said on Wednesday it would consider "constructive proposals" to end the bloodshed in Syria but opposed the use of force or sanctions.
Foreign Minister Sergei Lavrov said any UN Security Council resolution backed by Moscow "must firmly record that it cannot be used or interpreted to justify anyone's outside military intervention in the Syria crisis."
Russia and China both blocked a previous Western attempt to have the Security Council formally condemn Assad's crackdown and impose stiff sanctions if he refuses to enter direct talks.
According to diplomats at the United Nations, European and Arab nations are currently hammering out a Security Council draft resolution condemning the crackdown.
They are said to want a vote early next week on a resolution that also hints at sanctions.
A first draft of the new resolution, obtained by AFP, notes Arab League sanctions ordered against Syria and "encourages all states to adopt similar steps and fully to cooperate with the League of Arab States in the implementation of its measures."
On the ground, Syrian security forces continued their campaign against the central city of Hama, long a hotbed of resistance against Assad's rule.
"The Syrian army is bombarding Hama with heavy weapons, using rocket-propelled grenades," said a statement on Wednesday from the Local Coordination Committees, which organises anti-regime protests on the ground.
Al-Watan newspaper, which is close to the authorities, said the military had launched an offensive aimed at retaking several districts in Hama that were controlled by insurgents.
"The competent authorities have decided to resolve the matter in a definitive manner in order to relieve the city of armed militias," it said.
Meanwhile, the International Committee of the Red Cross has demanded a probe into Wednesday's murder of top aid worker Abdelrazak Jbeiro in the northwestern province of Idlib.
It said Jbeiro, the Syrian Red Crescent's secretary general and president for Idlib, was shot dead near Khan Sheikun on the Halab-Damascus highway.
The Red Cross said the Syrian Arab Red Crescent president, AbdulRahman al-Attar, had "officially requested the Syrian authorities to launch an investigation into the death of Dr Jbeiro."
On the economic front, Canada on Wednesday announced new sanctions against four banks, three oil companies and 22 people linked with the Assad regime -- Ottawa's fifth round of sanctions against Syria since May.
"We want to make sure we are doing everything we can to isolate this reprehensible regime," Foreign Minister John Baird said in a statement.
"Assad and those supporting him must get the message that peace-loving nations of the world are working together to end the regime's oppressive rule," he added.
Canada has already banned imports from Syria and new investment there, and imposed an assets freeze prohibiting economic dealings with individuals and entities associated with the Assad regime.