The U.N. humanitarian office's operations director says the Syrian president's regime has refused entrance visas to aid workers from Britain, Canada, France and the United States.

John Ging says "tremendous political obstruction" by Bashar Assad's government has hindered the United Nations' and other international organizations' aid efforts.

He told reporters Monday in Geneva that his agency, however, enjoys "a positive engagement" with Assad's government, with which a written deal "is being implemented" to allow aid workers and supplies to enter four hard-hit provinces.

Radhouane Nouicer, the U.N.'s regional humanitarian coordinator for Syria, says much of the help for the four provinces - Daraa, Deir el-Zour, Homs and Idlib - is held up by visa delays and other bureaucratic hurdles.