The UN's humanitarian chief, Baroness Amos, is to travel to Syria to negotiate access for aid workers in the conflict-torn country as the reported death toll rises.
It comes amid reports of continuing violence and increasing concerns of a humanitarian disaster in the country, and on the day it emerged Sunday Times reporter Marie Colvin and French photographer Remi Ochlik had been killed in an artillery attack in the city of Homs.
The UN said Baroness Amos, a former leader of the House of Lords, would be dispatched to Syria "to assess the humanitarian situation and renew the call for humanitarian access" to help residents trapped in conflict zones.
Meanwhile, the international community remains divided on how to tackle the crisis.
Russian president Dmitry Medvedev and his Iranian counterpart Mahmoud Ahmadinejad have rejected foreign intervention, the Kremlin said.
In a telephone call on Wednesday the pair - whose countries are the Assad regime's closest allies - reportedly said they favoured a peaceful resolution of the crisis "by the Syrian people themselves".
Human rights groups have made repeated calls for a ceasefire so help can be given to civilians caught up in the violence. Activists say residents need food, medical care and other provisions.
But forces loyal to Syria's president, Bashar al Assad, have continued to bombard Homs with rockets and bombs, reducing buildings to rubble and reportedly killing more than 20 people on Wednesday - including Ms Colvin and Mr Ochlik.
The city has been subjected to a three-week onslaught from government troops, and is now the focal point of a nationwide uprising against Mr Assad's 11-year rule.
Opposition group the Syrian National Council said it is coming to the view that foreign military intervention is the only way to end the deadlock, having previously argued that the Syrian people had to fight their own battles.
The US has made the strongest hint yet that it could arm the opposition forces of the Free Syrian Army, with Washington saying in a statement: "We don't believe that it makes sense to contribute now to the further militarisation of Syria. What we don't want to see is the spiral of violence increase.
"That said, if we can't get Assad to yield to the pressure that we are all bringing to bear, we may have to consider additional measures."
Britain's Foreign Office earlier summoned Sami Khiyami, Syria's ambassador to London, for a meeting in which it was stressed that the Government was "horrified" by "unacceptable" violence in Homs.
A Foreign Office spokesman said it was made clear that on Wednesday alone "the world had witnessed the death of more than 60 civilians, including children, on the single street of al Hakoura in the Baba Amr neighbourhood".
This week an international conference in Tunisia on how to end the bloodshed will be attended by the Syrian opposition and not the regime.
Russia announced it would not attend the "Friends of Syria" meeting because it was being convened "for the purpose of supporting one side against another in an internal conflict".
US Secretary of State Hillary Clinton, who will participate, said Syria was increasingly under pressure.
The Friday meeting will "demonstrate that Assad's regime is increasingly isolated and that the brave Syrian people need our support and solidarity," she said.
The UN estimates at least 5,400 civilians have been killed since the beginning of protests against the Assad regime last March.
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