The US defence department has said it is examining an Islamic State video appearing to show militants in control of US weapons intended for Syrian Kurdish fighters.
Some 27 bundles containing small arms, ammunition and other weaponry were dropped on Monday for militias defending the town of Kobane from IS.
A Pentagon spokesman said the vast majority ended up in the right hands.
Kurdish forces control most of the town but IS remains a threat, he said.
Militants launched a fierce attack “on all fronts” in Kobane on Monday after two days of relative calm when the town’s defenders appeared to have pushed them back.
But Pentagon spokesman Rear Adm John Kirby said IS had been kept at bay by a combination of US-led air strikes and the efforts of the Kurdish forces.
The British-based Syrian Observatory for Human Rights said that 30 IS fighters and 11 Kurdish defenders were killed on Tuesday, adding that the Islamists were bringing reinforcements.
Kobane, on the Turkish border, has been been under assault from IS for weeks, with most civilians forced to leave.
The new fighting came as Turkey said it would allow Iraqi Kurdish Peshmerga fighters to cross into Syria to fight IS.
However, a local Kurdish official, Idris Nassen, said he did not have “any idea” when this might happen, the AFP news agency reported.