Human Rights Watch called on international donors gathering for a Syria aid conference in Kuwait Wednesday to demand access for relief deliveries to civilians trapped by the fighting.
"Syrian authorities have... been unwilling to allow access into besieged areas or civilians to leave towns where an estimated 288,000 people are trapped with little or no aid," the New York-based group said on Tuesday.
Several battleground areas of Damascus province and Homs province to its north have been under siege for at least a year.
Activists have given harrowing accounts of food and medical shortages in besieged areas, including the Old City of Homs and the Yarmuk Palestinian refugee camp in south Damascus.
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HRW said the Syrian authorities have taken some steps towards facilitating greater aid access.
But "rather than congratulating Syria on taking small steps that are years overdue, donors should be demanding immediate access to besieged towns and supporting cross-border aid from Turkey," said its global advocacy director Peggy Hicks.
Though some aid has been allowed into Syria via Iraq, Jordan and Lebanon, Damascus has "steadfastly refused to allow aid from Turkey to reach those in need in northern Syria," said HRW.
Ankara has been a leading backer of the opposition and many areas bordering Turkey are under rebel control.
HRW said Syrian government allies "Russia and Iran, in particular, should press Syria to eliminate obstacles to humanitarian aid."
The United Nations is seeking an unprecedented $6.5 billion in aid at the Kuwait donors' conference to assist 9.3 million Syrians inside their country and well over two million refugees.