Azaz, which is home to around 35,000 people, is also the town where rebels have been holding 11 Lebanese Shiites they captured in May. On Wednesday, Lebanese media reported conflicting reports on their fate, but it was unclear whether they had been affected by the bombing.
"All parties must do more to protect civilians," said Amos at the end of the three-day mission to try to open more channels for international aid inside Syria.
She observed that "the humanitarian situation has worsened" since her last visit to Syria in March, when the U.N. estimated more than 1 million people had been displaced or in need of critical humanitarian aid. "Now as many as 2.5 million are in need of assistance," she said.
Later in Beirut, Amos expressed frustration at Syria's reluctance to allow more major international aid groups into the country because of Syrian fears that relief supplies could reach rebels.
"They don't want to see that happen," she said.
France's foreign minister said that while humanitarian aid was needed, "we also need political action to achieve replacing Bashar Assad, and we need action on the ground carried out by the rebel army."