The cache of weapons included hand grenades, ammunition and rocket-propelled grenade launchers, according to a video uploaded by a media group loyal to the Islamic State.
The video appeared authentic and corresponded to The Associated Press' reporting of the event. The Britain-based Syrian Observatory for Human Rights said the militants had seized at least once cache, but may have seized more.
US military forces carried out four air strikes on Islamic State targets in Syria on Monday and Tuesday and were joined by partner nations in three attacks in Iraq, the US Central Command said.
Fighter, bomber and attack aircraft were used in the raids and all returned safely, the Central Command statement said.
The strikes near Kobani, Syria, destroyed Islamic State fighting positions, a building and an Islamic State unit.
In Iraq, a fighting position southeast of the Mosul Dam and one south of the Bayji oil refinery were destroyed while another strike north of Fallujah suppressed an Islamic State attack, the statement said.
The Observatory, which bases its information on a network of activists on the ground, said the caches were airdropped early Monday to Kurds in the embattled Syrian town of Kobani that lies near the Turkish border. The militant group has been trying to seize the town for over a month now, causing the exodus of some 200,000 people from the area into Turkey. While Kurds are battling on the ground, a US-led coalition is also targeting the militants from the air.
On Tuesday, Islamic State loyalists on social media posted sarcastic thank you notes to the United States, including one image that said, "Team USA."
But the badly-aimed weapons drop was more an embarrassment than a great strategic loss. The Islamic State militants already possess millions of dollars-worth of U.S. weaponry that they captured from fleeing Iraqi soldiers when the group seized swaths of Iraq in a sudden sweep in June.
Also Tuesday, Syrian government airstrikes hit a rebel-held town along the country's southern border with Jordan, killing at least eight people on Tuesday.
Activists with the Local Coordination Committees and the Britain-based Observatory said the number of those killed was likely to rise as there are more victims under the rubble.
The LCC said Syrian government planes dropped crude explosives-laden canisters on the town of Nasib on the Syria-Jordan border.
The airstrikes are part of battles between Syrian government forces and Islamic rebel groups for control of the area.
Syrian government forces have been heavily bombing rebel areas in recent weeks, while the U.S-led coalition has been conducting airstrikes against Islamic State militants elsewhere in Syria.
Reuters contributed to this report