Last week, warlord Haftar's LNA sent militants and weapons to bolster its defence of Sirte, already badly battered from warfare and chaos since the 2011 revolution.
Its Africa Command said on Friday there was mounting evidence from satellite pictures of Moscow's military cargo planes, including IL-6s, bringing supplies to fighters from the Russian Wagner Group.
Both sides have been mobilising forces around Sirte, where any major new escalation could risk drawing major regional powers further into Libya's messy conflict.
The Tripoli-based internationally recognised Government of National Accord (GNA) is backed by Turkey. The eastern-based militias (LNA) of warlord Khalifa Haftar are backed by Russia, the United Arab Emirates and Egypt.
READ MORE: Turkey, Russia agree to push for Libya truce
LIBYA: #Russia & the #Wagner Group continue to be involved in both ground & air operations in #Libya— US AFRICOM (@USAfricaCommand) July 24, 2020
"The type and volume of equipment demonstrates an intent toward sustained offensive combat action capabilities, not humanitarian relief," said Gering.
"The type and volume of equipment demonstrate an intent toward sustained offensive combat action capabilities," the Africa Command said in a statement posted on its website.
Both Russia and its ally have denied previous US military statements that Moscow has sent fighter jets to back Wagner forces there.
The GNA earlier this year pushed Haftar's militias from most of the territory it held in northwest Libya, including in Tripoli, destroying several Russian air defence systems.
However, the warlord's militias stopped retreating at the central coastal city of Sirte, which it took from the GNA in January, and the front line has solidified there.