Ban met the head of the investigation team, Ake Sellstrom, as international suspicions about the use of the weapons grow and on the day designated to remember the victims of chemical weapons attacks.
Ban told reporters he "takes seriously" US reports about the weapons and said "I again urge the Syrian authorities to allow the investigation to proceed without delay and without any conditions."
Sellstrom and an advanced team now in Cyprus can deploy to Syria "within 24 to 48 hours," the UN secretary general said.
Britain and France have asked that the inquiry also look at opposition claims that chemical arms also had been used in Homs and near Damascus.
Ban wrote a new letter to President Bashar al-Assad on Thursday seeking access as the United States revealed its suspicions that chemical arms have been used. Diplomats said the Syrian government is barely communicating with UN and other international bodies.
"This is a crucial moment in our efforts to get the team on the ground to carry out its important task," Ban said.
"I take seriously the recent intelligence report of the United States about the use of chemical weapons in Syria. On-site activities are essential if the United Nations is to be able to establish the facts and clear up all the doubts surrounding this issue.
"A credible and comprehensive inquiry requires full access to the sites where chemical weapons are alleged to have been used," he added.
