U.S. military officials say they are investigating claims that Islamic State fighters seized a cache of weapons that were airdropped by U.S.-led coalition forces and meant for Kurdish militiamen.

A video uploaded by a media group loyal to the Islamic State showed a cache of weapons including hand grenades, ammunition and rocket-propelled grenade launchers. Their claims would appear to correspond with those of the Britain-based Syrian Observatory for Human Rights, which said the militants had seized at least one cache.

U.S. officials said Tuesday they could not confirm the video's authenticity but are looking into the claims.

"We just don't know. We're still looking at it," Pentagon spokesman Rear Adm. John Kirby said when asked about the video, while stressing that the "vast majority" of supplies ended up in the right hands.

The supplies were airdropped early Monday to Kurds in the embattled Syrian town of Kobani that lies near the Turkish border. The militant Islamic State 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 U.S.-led coalition is also targeting the militants from the air.

In all, 28 bundles of equipment -- including medical supplies, weapons and ammunition -- were dropped. The Pentagon says 27 bundles were received.

According to Pentagon spokesman Col. Steve Warren, "one of those bundles drifted off course." But he said "we subsequently destroyed it with an airstrike."

On Tuesday, though, Islamic State loyalists on social media posted sarcastic thank you notes to the United States, including one image that said "Team USA."

Kirby acknowledged the conflicting accounts. "We're aware that one bundle did not make it into the right hands and you saw the CENTCOM release indicating that they destroyed it from the air," he said. "All of that doesn't take away from the notion that this video is out there and that it could in fact be that bundle. We just don't know."

The lost 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.

On Tuesday, U.S. Central Command said U.S. military forces conducted four airstrikes near Kobani that destroyed Islamic State fighting positions, a building and a large Islamic State unit.

Also Tuesday, Syrian government airstrikes hit a rebel-held town along the country's southern border with Jordan, killing at least eight people.

Activists with the Local Coordination Committees and the 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.

Fox News' Justin Fishel and Greg Palkot and The Associated Press contributed to this report.