In his monthly report to the UN Security Council, Ban said that a total of 4.7 million Syrians are now in areas that are "difficult or impossible" for humanitarian workers to reach, including 241,000 in besieged areas.
That's an increase from the previous estimate of 3.5 million people. The report warned that advances made by Islamic State of Iraq and the Levant militants in Iraq would only further worsen the situation in Syria.
"Far from improving access, new procedures rolled out two months ago have resulted in more delays and reduced the reach of humanitarian partners further."
Of the 10.8 million total people in need of aid -- a 17 percent, or 1.5 million increase from previous estimates -- about 6.4 million are internally displaced.
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An estimated 160,000 people have been killed in the conflict.
The report comes as Western powers work toward a new Security Council resolution that would guarantee the delivery of aid.
The letter, signed by Syrian and Arab lawyers, warned that the state must give consent for imports of any humanitarian aid.
"Importing aid in coordination with terrorist organisations and without consultation with the Syrian state would amount to an attack on the Syrian state," said the letter.
It claimed it could be used as a "pretext for aggression."
Syria opposes the delivery of cross-border aid that would send supplies directly to areas held by the armed opposition in its more than three-year brutal civil war.