The Pentagon claimed American-supplied weapons were not used to attack a Russian airbase in Crimea, which voted in 2014 to break off from Ukraine and join Russia.
Several explosions took place on Tuesday at the Saki base in Crimea, which damaged or destroyed some eight aircraft, as well as ammunition stockpiles. Ukrainian forces are believed to be behind the blasts.
The US Defense Department said on Friday that it did not know the cause of the devastating explosions at the site, AFP reported.
No one has claimed responsibility and it is also not clear what caused the blasts at the base.
Russia has said the incident was an accident, but American media claimed that it was an attack.
The Pentagon has supplied large amounts of weaponry and ammunition to Ukraine. The US military has also confirmed the delivery of American anti-radar missiles to Ukraine in a bid to facilitate the targeting of Russian radar systems by Ukrainian warplanes.
"We haven't provided anything that allows or that would enable them to strike into Crimea," a senior US defense official told reporters.
The official specifically said it was not a US precision-guided, medium-range tactical missile, known as Army Tactical Missile System (ATACMS) missiles. Ukraine has requested ATACMS which can be launched by US-supplied HIMARS systems already in Ukraine.
"It was not ATACMS, because we have not given them ATACMS," the defense official said.
The official added that the US military did not know whether it was a missile attack on the base, or whether it was an act of sabotage.
The official claimed it had a "pretty significant impact on Russian air (operations) and air personnel."
British intelligence officials said Friday that the numerous huge explosions recorded on video by people in the area "were almost certainly from the detonation of up to four uncovered munition storage areas."
They claimed that five Su-24 fighter-bombers and three Su-30 multi-role jets were destroyed or seriously damaged by the explosions.
"Saki was primarily used as a base for the aircraft of the Russian Navy's Black Sea Fleet," British defense Intelligence said.
"The fleet's naval aviation capability is now significantly degraded," it added.
US Defense Department's Undersecretary for Policy Colin Kahl declared at a press briefing on Monday that Pentagon had shipped "a number" of the missiles to Ukraine without elaborating on how many and when they had been sent.
Citing a military official, however, CNN identified the type of the missile sent as "the AGM-88 High-Speed Anti-Radiation Missile (HARM)," marking the first time the Pentagon has acknowledged sending the previously undisclosed missile to Ukraine.
Kahl unveiled the shipment of the missiles to Ukraine in a briefing about Washington's latest $1-billion weapons package for Kiev, the largest package of US weapons to date.
With the latest package, the US has committed to sending Ukraine a total of $9.1 billion in military assistance since the outset of Russia's "special military operation" in Ukraine in late February.
The package was focused on supplying ammunition for some of the key weapon systems the US has so far shipped to Ukraine. It includes the High-Mobility Artillery Rocket System (HIMARS), which the Ukrainian military has used to target what the US media describe as Russian command posts, ammunition depots, and more, as well as M777 howitzers, which have been a critical part of the battle in the Donbass region of eastern Ukraine.
The development came amid US press reports that Washington is planning to ship to Ukraine an additional $5.5 billion in military aid as it ramps up support for Kiev in its war with Russia.
Meanwhile, Russian forces have destroyed a US-made radar system and advanced US-made missiles in eastern Ukraine.
In its daily briefing, Russia's Defense Ministry said on Friday that two High Mobility Artillery Rocket System (HIMARS) and AN/MPQ-64 radar systems in use by Ukrainian forces were destroyed in the Donetsk region.
On Wednesday, the ministry said that Russian forces destroyed a German-supplied Gepard anti-aircraft system in use by Ukrainian forces in the Mykolaiv region.
The defense ministry also said it had shot down three Ukrainian warplanes in the Mykolaiv region, as well as seven HIMARS missiles in the neighboring Kherson region.
Russia has repeatedly warned that US and NATO shipments of weapons systems to Ukraine add fuel to the conflict between Moscow and Kiev and could bring unpredictable consequences.