Both men are residents of Alabama. Relatives of Andy Tai Ngoc Huynh, 27, of Trinity and Alexander Drueke, 39, of Tuscaloosa have been in contact with both Senate and House offices seeking information about the men's whereabouts, press aides said.
Rep. Robert Aderholt said Huynh had volunteered to go fight with the Ukrainian army against Russia, but relatives haven't heard from him since June 8, when he was in the Kharkiv region of northeastern Ukraine, which is near the Russian border. Huynh and Drueke were together, an aide to Aderholt said.
"As you can imagine, his loved ones are very concerned about him," Aderholt said in a statement. "My office has placed inquires with both the United States Department of State and the Federal Bureau of Investigation trying to get any information possible."
The U.S. State Department said it was looking into reports that Russian or Russian-backed separatist forces in Ukraine had captured at least two American citizens. If confirmed, they would be the first Americans fighting for Ukraine known to have been captured since the war began Feb. 24.
"We are closely monitoring the situation and are in contact with Ukrainian authorities," the department said in a statement emailed to reporters. It declined further comment, citing privacy considerations.
Two Americans fighting alongside Ukrainian forces are missing
Huynh's fiancee, Joy Black, posted publicly on Facebook that his family was in contact with the Drueke family and government officials, and that nothing had been confirmed other than that the two were missing.
"Please keep Andy, and Alex, and all of their loved ones in prayer. We just want them to come home," she wrote.
U.S. Rep. Adam Kinzinger tweeted that the Americans "have enlisted in the Ukrainian army, and thus are afforded legal combatant protections. As such, we expect members of the Legion to be treated in accordance with the Geneva convention." It was unclear whether Kinzinger, an Illinois Republican, had any further information about the men.
He was commenting on a tweet sent earlier Wednesday by Task Force Baguette, a group of former U.S. and French servicemen, saying that two Americans fighting with them were captured a week ago. The group said Ukrainian intelligence confirmed the information.
Early in the war, Ukraine created the International Legion for foreign citizens who wanted to help defend against the Russian invasion.
Huynh, an Orange County native, spoke to his local newspaper in Alabama, the Decatur Daily, shortly before flying to Eastern Europe in April.
He explained that he was studying robotics at Calhoun Community College but couldn't stop thinking about Russia's invasion.
"I know it wasn't my problem, but there was that gut feeling that I felt I had to do something," Huynh told the Decatur Daily. "Two weeks after the war began, it kept eating me up inside and it just felt wrong. I was losing sleep. ... All I could think about was the situation in Ukraine."
He said he decided to fly out once he learned that young Ukrainians were being drafted into service.
"Right when they turned 18, they were forced to enlist in the military to defend their homeland," Huynh said. "Honestly, that broke my heart. I would say that is probably the moment where I decided that I have to do something."
According to the newspaper, Huynh enlisted in the Marines when he was 19 and served for four years, though he did not see active combat.
He was born and raised in Orange County to Vietnamese immigrants and moved to northern Alabama two years ago to be closer to his fiancée, the newspaper reported.
Members of a church in Alabama gathered last April to wish him well before he left for Ukraine.
John Kirby, a national security spokesman at the White House, said Wednesday that the administration wasn't able to confirm the reports about missing Americans.
"We'll do the best we can to monitor this and see what we can learn about it," he said.
However, he reiterated his warnings against Americans going to Ukraine.
"Ukraine is not the place for Americans to be traveling," he said. "If you feel passionate about supporting Ukraine, there's any number of ways to do that that that are safer and just as effective."
The Associated Press contributed to this report.