"We are very confident that the vast majority of the bundles did end up in the right hands except one," Pentagon spokesman John Kirby told journalists Tuesday.
Kirby added that he could not confirm that the missing bundle was seized by the IS after its drop by US planes over the northern Syrian city of Kobane.
On Tuesday, IS fighters released a video on YouTube in which they appear with boxes wrapped in the net of a large parachute which they later open and find military material, including grenades.
On Sunday, the US airdropped arms, ammunition and medical supplies to the Kurdish fighters combating the IS in Kobane.
The Syrian Observatory For Human Rights, which monitors events in the country, said that two parachutes carrying arms and medical supplies landed in the IS-controlled area.
According to the London-based NGO, the extremists took charge of one of the parachutes, but the second one was destroyed by international coalition forces before the radicals could take the weapons and other material.
Kirby said that most of the city was under the control of Kurdish forces, but the IS remains a threat and it was still not known if more airdrops would be necessary.
Kobane, one of the three main Kurdish settlements in Syria, is surrounded by jihadis from all sides except in the north, where it shares the border with Turkey.