U.N. humanitarian chief Valerie Amos says a United Nations aid mission to the southern Syrian city of Daraa has been postponed until later this week because the country failed to provide access.

U.N. spokesman Farhan Haq said Monday that Syrian President Bashar al-Assad had indicated a "willingness" to give the humanitarian mission access to Daraa during a telephone conversation last week with Secretary-General Ban Ki-moon. But Haq said as of Monday, no access had been given.

Daraa is a key city where armed Syrian forces have put down anti-government demonstrations influenced by the wave of protests in the Arab world.

Amos said later Monday that she's trying to find out why the mission was not allowed in Sunday as scheduled.