The International Committee of the Red Cross (ICRC) says the Syrian government and rebel fighters have agreed a temporary truce to allow civilians to be evacuated from the city of Homs.
The ICRC said that its teams - with the Syrian Arab Red Crescent - were ready to enter the worst-hit parts of Homs.
However despite the agreement, aid workers have been unable to enter.
Elsewhere in Syria, activists say that at least 20 soldiers were killed by rebels in the province of Latakia.
The UK-based Syrian Observatory for Human Rights said the fighting took place in a region known as Jabal al-Akrad near the Turkish border.
Homs has been at the centre of the 15-month revolt against President Bashar al-Assad.
Beatrice Megevand-Roggo, the ICRC's regional head of operations, said fighting had been raging for more than 10 days in parts of the city.
"Hundreds of civilians are stuck in the old city, unable to leave and find refuge in safer areas," she said.
The ICRC said it had received promises from both sides that they would allow a two-hour pause in the fighting. The ICRC says it is essential it be given immediate access.
But although aid workers stood ready on Wednesday, they were not able to enter the city. It was not clear whether the heavy shelling and gunfire had stopped.
Heavy shelling by government forces in residential areas has killed hundreds of civilians and brought international condemnation.
The Syrian government blames the deaths on "terrorists" backed by foreign powers.
On Tuesday, the government said it was trying to evacuate civilians and blamed rebel fighters for obstructing its efforts.
UN mission 'staying'
Earlier on Wednesday, the Syrian Observatory for Human Rights said that at least 20 soldiers were killed in an overnight attack by rebels on an army barracks in Latakia.
It said that dozens of soldiers had been wounded and others - including an officer - captured.
The group added that 58 people had been killed in violence across the country on Wednesday.
Neither the reports of attacks nor the casualty figures can be independently verified.
Latakia has recently seen some of the worst of the fighting.
Last week, the Syrian government said it had taken control of the western mountain town of Haffa, in Latakia, after fierce fighting with rebels.
It said Haffa had been "purged of terrorists" and calm restored.
UN monitors were fired on and turned away by angry residents as they tried to enter the town amid fears of a massacre. They were able to return on Friday.
The UN observer mission has now suspended its operations in Syria amid the worsening violence.
Its leader, Maj Gen Robert Mood, told the UN Security Council that observers were targeted by hostile crowds and gunfire on at least 10 occasions, diplomats said.
But he said his decision to suspend operations did not mean the mission was leaving Syria or abandoning the Syrian people.
The UN mission has been observing the implementation of a peace plan brokered by UN-Arab League envoy Kofi Annan.
However, a ceasefire that was supposed to have come into effect in April never took hold.
Meanwhile, diplomatic efforts to end the violence have been continuing.
At the G20 summit in Mexico on Tuesday, US President Barack Obama held separate talks with the leaders of Russia and China - both of which have blocked UN resolutions over Syria.
He said later that the talks had failed to achieve a breakthrough.
(Source : BBC)