The US defence department has said it is examining an Islamic State video appearing to show militants in control of 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.
On Monday, the US military said it had carried out airdrops of weapons, ammunition and medical supplies to the Kurdish fighters.
It said that 27 bundles had been dropped. A later statement said one bundle went astray but was destroyed to prevent it falling into enemy hands, and that all the others were safely delivered.
But a video uploaded to the internet by a media group loyal to IS showed a cache of weapons, apparently dropped by the US, in militant hands.
Activists from the Syrian Observatory for Human Rights also said militants had seized one cache.
Rear Adm Kirby said he could not confirm that the video was authentic.
"They are certainly of the kinds of material that was dropped... so it's not out of the realm of the possible in that regard," he said.
"When we have something definitive that we can provide in terms of an assessment on that we'll do that," he said.
The IS advance in Syria takes place against the backdrop of the civil war. US-led air strikes are being conducted there without the permission of President Bashar al-Assad, who the West wants to relinquish power.
In Iraq, the air campaign is taking place with the co-operation of the government. The advance of IS there earlier this year has taken it to close to the capital, Baghdad.
It still threatens the city, having taken over most of the western Anbar province in recent weeks.
IS is thought to be responsible for a series of bombings in Baghdad over the past few days which have left dozens of people dead.
On Tuesday at least 12 were killed in a double car bomb attack in the mainly Shia Talibiya district of eastern Baghdad.
Correction: An earlier version of this article wrongly referred to the contents of the airdrops in Kobane as "US weapons". The weapons were in fact supplied by Kurdish authorities in Iraq.
- Formed out of al-Qaeda in Iraq (AQI) in 2013, IS first captured Raqqa in eastern Syria
- It captured broad swathes of Iraq in June, including Mosul, and declared a "caliphate" in areas it controls in Syria and Iraq
- Pursuing an extreme form of Sunni Islam, IS has persecuted non-Muslims such as Yazidis and Christians, as well as Shia Muslims, whom it regards as heretics
- Known for its brutal tactics, including beheadings of soldiers, journalists and aid workers
- The CIA says the group could have as many as 31,000 fighters in Iraq and Syria