The UN Security Council is demanding immediate access to Syria for its humanitarian chief, Valerie Amos.
Russia and China, who vetoed two previous Security Council resolutions on Syria, are also backing the call for Baroness Amos to be allowed in.
It comes as the rebel Free Syrian Army said it was withdrawing from the Baba Amr district of Homs, which has been under siege for nearly a month.
Activists say hundreds of people have died during the bombardment.
Earlier, the International Committee of the Red Cross (ICRC) said it and the Syrian Red Crescent had been given the green light to go to Baba Amr on Friday to deliver food and medical supplies.
In a statement, the council expressed its "disappointment" that Baroness Amos had not been granted authorisation to visit Syria.
The council's 15 member countries also said in a unanimously agreed statement that they "deplore" the deteriorating situation, "in particular the growing number of affected civilians, the lack of safe access to adequate medical services, and food shortages, particularly in areas affected by fighting and violence such as Homs, Hama, Deraa, Idlib".
"The members of the Security Council call upon the Syrian authorities to allow immediate, full and unimpeded access of humanitarian personnel to all populations in need of assistance, in accordance with international law," the statement said.
On Wednesday, Baroness Amos, who heads the UN Office for the Co-ordination of Humanitarian Affairs (UNOCHA), said she had been refused permission by the Syrian authorities to enter the country.
Russia and China vetoed earlier resolutions on Syria at the Security Council out of concern that they could lead to Libya-style regime change.
The UN estimates more than 7,500 people have died in an 11-month anti-government uprising in Syria.
The Local Co-ordination Committees, a network of activists, says 33 people died on Thursday, including 21 in Homs. Their figures cannot be verified independently.
The ICRC and Syrian Red Crescent plan to bring bring food and medical supplies to Baba Amr on Friday, and will also evacuate those in need, ICRC spokeswoman Carla Haddad Mardini told the BBC.
"We fear there are many people who are seriously wounded," she said. "We know the humanitarian situation on the ground is extremely worrying."
Earlier on Thursday the FSA, the main armed rebel group in the country, said it was making a "tactical withdrawal" from Baba Amr.
It came after nearly a month of bombardment and the first stages of a ground offensive by Syrian government forces.
Of the 100,000 people who normally live in Baba Amr, only a few thousand remain. The FSA said 4,000 civilians had refused to leave their homes and it was withdrawing to save them from an all-out assault.
It has been snowing heavily in Homs, slowing the advance of ground troops which began their offensive on Tuesday, but also worsening conditions for civilians.
Many of those still in Baba Amr are without power and running low on basic supplies.
Meanwhile, the exile political opposition Syrian National Council (SNC) says it has formed a military bureau to co-ordinate the various armed anti-government groups.
Announcing the creation of the new bureau, SNC leader Burhan Ghalioun said the uprising had begun as a non-violent movement, but the council had to "shoulder its responsibilities in light of this new reality".
Mr Ghalioun said the bureau would function like a defence ministry and would be staffed by soldiers from the FSA as well as civilians.
However, the head of the FSA, Col Riyad al-Asaad, has said his organisation will not co-operate with the new bureau, our correspondent says.