Washington (CNN) Russia has deployed military forces to Libya that are "de-stabilizing" the North African country, a top State Department official warned Tuesday.

David Schenker, the State Department's assistant secretary for near eastern affairs, told reporters that Russia is deploying troops in "significant" numbers to support Libyan Gen. Khalifa Haftar , whose Libyan National Army launched an April offensive to seize Tripoli from the UN-backed Government of National Accord.

US officials have previously told CNN that they believe hundreds of Russian mercenaries affiliated with the Wagner Group, a paramilitary organization with ties to the Kremlin, have been in Libya for some time, working on behalf of Moscow and helping Haftar in his bid to capture Tripoli, however Schenker said that Russia is now deploying regular uniformed personnel there as well.

"Russian regulars are being deployed in significant numbers to support the LNA," Schenker said, describing their presence as "incredibly destabilizing." The presence of the Russian troops "raises the specter of large-scale casualties among the civilian population," Schenker said.

His comments come days after a senior US delegation met with Haftar on Sunday to "discuss steps to achieve a suspension of hostilities and a political resolution to the Libyan conflict," according to a State Department statement issued Monday.

The delegation included White House Deputy National Security Adviser for Middle Eastern and North African Affairs Victoria Coates, as well as senior representatives from the State Department, Energy Department and the US military.

"The officials underscored the United States' full support for the sovereignty and territorial integrity of Libya and expressed serious concern over Russia's exploitation of the conflict at the expense of the Libyan people," the statement added.

Schenker defended meetings between US officials and Haftar, arguing that such meetings have long taken place. "The significance is that we're dealing with him at higher levels," Schenker said.

"We are engaged in trying to find a diplomatic solution ... we as a matter of policy talk to everyone," Schenker said, adding that US government officials have long talked to Haftar. "He has armor and significant forces under his control," the assistant secretary said, noting that Haftar controls 80% of the country. "We meet with everyone. ... This is nothing new."

The US has become increasingly vocal about its opposition to Russia's presence in Libya, believing Moscow is seeking to establish a position there in part to challenge NATO's southern flank.

Possible Russian involvement in drone downing

The US military believes that Russian mercenaries backing Haftar may have been responsible for downing a US military drone Thursday while it was flying over Tripoli last week, two US defense officials told CNN.

A third official said forces aligned with Haftar were responsible for bringing down the aircraft but said it was unclear whether Russian contractors were responsible. The official said that Haftar's representatives have apologized for the downing of the drone.

Asked if Russian mercenaries were responsible, Nate Herring, an Africa Command spokesman told CNN in an email, "We are unable to provide further comment since this incident is under investigation."

Africa Command had previously said that drone operations there are intended "to counter terror activity in Libya and are fully coordinated with appropriate government officials." The US military has previously targeted ISIS and al Qaeda linked fighters in drone strikes in Libya.

Drone strikes are increasingly a part of the battle for Tripoli with both Libyan factions using them. The UN's mission in Libya said earlier this month that "drone strikes in support of Libyan National Army forces at well above 800 since the beginning of the conflict," using the official name of Haftar's faction.

The total number of drone strikes in support of the Government of National Accord is estimated at around 240, according to the UN.

It is possible that Haftar's forces and their Russian allies downed the US drone because they believed that it belonged to the Government of National Accord.

While Haftar has received backing from Russian contractors, Egypt and the UAE, the Trump administration -- despite officially backing the government in Tripoli -- has offered praise for Haftar in the past.

The Government of National Accord is also supported by Turkey and some US officials believe Turkey has supplied drones to enable it to conduct airstrikes against Haftar's forces.

"It is our judgment that the drone infrastructure and operations are facilitated by external parties to the conflict," the head of the United Nations Support Mission in Libya, Ghassan Salamé, told the UN Security Council earlier this month.