Following the declaration of two US citizens that have gone missing in Ukraine, presumably captured by Russian forces, State Department spokesperson Ned Price that there were "reports of one additional American whose whereabouts are unknown."

On Thursday, Price spoke to the media and clarified that "Unfortunately we don't know the full details of that case."

While CNN reported on the identity of the third missing American as being the US Marine veteran Grady Kurpasi, based on information collected from his wife, the US official did not validate the information.

On Thursday, Russian Foreign Ministry spokesperson Maria Zakharova told the Solovyov Live show, when asked about the two captured US veterans in specific: “I do not have such information, and I have been checking it every day. I will check it today as well. We disclose to the public all information about the fate of arrested mercenaries or those on trial... I want to reiterate that I will double-check whether there is any additional information on this issue.”

Read more: Ukraine must find a diplomatic solution: US officials

On Wednesday, The Telegraph reported that these are the first two American mercenaries captured in Ukraine by Russian forces. The two captives, according to the newspaper, are former US servicemen, Alexander Drueke, 39, and Andy Huynh, 27. 

US mercenaries in Ukraine

Ukrainian news agency Ukrinform reported that 3,000 US volunteers are ready to arrive in Ukraine, amid Russia's ongoing military operation in the country.

The agency quoted the Ukrainian Armed Forces’ press service via Telegram as saying that "the volunteers are ready to repel Russia as part of an international battalion."

Among the US volunteers are army veterans "with combat experience gained in Iraq and other hotspots around the world."

Earlier in February, President Volodymyr Zelensky urged foreigners to head to Ukrainian embassies worldwide to sign up for an "international brigade" of volunteers to help fight against the Russian operation in Ukraine. 

"All foreigners wishing to join the resistance against the Russians occupiers and protect global security are invited by the Ukrainian leadership to come to our state and join the ranks of the territorial defense forces," Zelensky said in a statement.

"A separate unit is being formed from foreigners -- the International Brigade of the territorial defense of Ukraine. This will be a key testimony of your support for our country."

Russian Defense Minister Sergei Shoigu previously stated that there are more than 6,000 foreign mercenary fighters in Ukraine, adding that the West refuses to recognize many facts of torture and brutal killing of Russian prisoners of war, noting that Russian forces, unlike the armed forces of Ukraine, do not strike civilian infrastructure.

Most recently, high-ranking White House officials argued that the conflict in Ukraine must have a diplomatic solution, following increasing concerns about Kiev's deteriorating situation on the battlefield.

Read more: Zelensky: 16,000 foreign mercenaries are going to fight for Ukraine