"We are watching a humanitarian tragedy unfold before our eyes," Amos told a news briefing. "We must do all we can to reassure the people that we care and that we will not let them down."
Syrian opposition representatives told the United Nations this week that some three million people living throughout rebel-held territory require international assistance, she said.
The Syrian government still refuses to allow U.N. convoys to cross from Turkey into northern Syria, as most border crossings are controlled by the Free Syrian Army, she said.
Four million Syrians were deemed in need of aid late last year, but the situation has deteriorated since due to shelling, inflation, and shortages of food and medicine, she said.
Some 70,000 people have been killed in the nearly two-year-old revolt against President Bashar al-Assad that has also sent 860,000 refugees fleeing abroad, according to the world body.
Typhoid has broken out in an rebel-held Deir al-Zor due to people drinking contaminated water from the Euphrates River, the World Health Organization (WHO) said on Tuesday.
In the last few weeks, the U.N. refugee agency reached rebel-held Azaz with aid for the first time. WHO has delivered vaccines in many opposition-held areas, Amos said.
"Cross-line operations are difficult but they are do-able.
"We are crossing conflict lines, negotiating with armed groups on the ground to reach more in need. But we are not reaching enough of those who require our help. Limited access in the north is a problem that can only solved using alternative methods of aid delivery," Amos said.
She held talks with Suhair al-Atassi, a vice president of the opposition Syrian National Council, on Monday. Syrian deputy foreign minister Hossam Eddin attended the closed-door Geneva forum on Tuesday but opposed the rebels taking part, she said.
The Syrian government has agreed that 3 more international agencies could deploy aid workers - Mercy Corps, NRC and Merlin - bringing the total to 11, but still not enough, Amos said.
"With respect to the Turkish border, I have spoken to the (Syrian) government on a number of occasions about allowing us to bring in supplies across that border. My last conversation with them was yesterday. The answer remains no," Amos said.
The U.N. must uphold General Assembly resolutions requiring consent of a government to allow relief goods to be imported, unless authorized by a Security Council resolution, she said.
It is importing fuel, vital for convoys and U.N. aid operations, via Lebanon with Syria's permission, she said.
LIMITED AND DIFFICULT ACCESS
John Ging, director of operations for the U.N. Office for the Coordination of Humanitarian Affairs, said that most goods are imported via Lebanon and Jordan, with Syria's permission.
"They are opposed to operations across borders that they don't control," Ging told Reuters in an interview on Tuesday.
"The reason it's significant and important for us is that our access across the conflict line from within to those areas (on the Turkish border) is very limited and very difficult and dangerous. As a result, we're not getting adequate or anything even close to an adequate humanitarian supply into those areas."
Claus Sorensen, director-general of the European Union Office for Humanitarian Aid who attended the Geneva forum, said it would be far better and safer to bring in aid through Turkey.
"This is very much what was the discussion was about today," Sorensen told reporters.
The United States, which announced a further $19 million aid contribution, called for "direct, cross-border delivery".
(Reporting by Stephanie Nebehay; Editing by Michael Roddy)
(This Feb 19 story was refiled to fix fix the name of the aid agency Merlin)