The UN under-secretary general for humanitarian affairs has expressed deep concern for communities cut off in Syria by months of fighting between government and rebel forces.
Valerie Amos told the BBC she had heard accounts of near starvation, including in the capital, Damascus.
Baroness Amos said she had spoken to the government about trying to get humanitarian access.
She said the situation in the country was getting worse by the day and she had heard horrific tales of suffering.
An international conference is due to begin in Geneva on 22 January to seek a political solution to the war which began in March 2011.
"The sick and wounded have not been able to leave, we've not been able to get food in,'' Baroness Amos said.
"There are reports of people on the brink of starvation including in the Yarmouk Palestinian refugee camp close to the centre of Damascus," she told the BBC.
On Friday, a UN official said residents in the Yarmouk camp, including infants and children, have been subsisting on diets of such things as stale vegetables, animal feed and cooking spices dissolved in water.
Christopher Gunness said infants there had diseases linked to severe malnutrition, including anaemia, rickets, and kwashiorkor (a protein deficiency).