Valerie Amos said the U.N. estimates that 4 million people in war-torn Syria are in need of some form of humanitarian assistance, the live blog of Britain's The Guardian reported.
In addition, the U.N. estimates some 2 million have been internally displaced while the death toll in the conflict is approaching 70,000.
Rebels and activist groups in Syria say insurgents have seized control of an important airfield and captured usable warplanes for the first time in two years.
Video clips uploaded on the Internet appear to confirm reports that al-Jarrah airfield in Aleppo Province is now in rebel hands, The New York Times reported.
It is unclear if any planes at the airfield suffered damage during the two-day surge by insurgents to capture al-Jarrah, the Times said.
Separately, there were unconfirmed reports late Tuesday of a mass defection by 40 members of Syria's Fourth Armored Division and the Republican Guard, two of the most loyal elements of President Bashar Assad's military.
The Local Coordination Committee, an anti-Assad activist group, posted a video on YouTube of the supposed defectors. If confirmed, the defection suggests the inner core of Assad's security apparatus may be starting to corrode.