Russian authorities say they have already started a probe into what made the two women -- Marina Kovalevsky and her daughter, Yana -- fall ill on February 24.
The women arrived in Los Angeles on March 7 and were admitted to a hospital in stable condition.
At a briefing in Washington March 8, U.S. State Department spokesman Sean McCormack said the United States was monitoring the case closely.
"As a U.S. government, we want to ensure that our citizens have answers to the questions that obviously come up -- how did this happen?" McCormack said. "And we will be checking in with Russian authorities as to the progress in their investigation."
The women were born in the Soviet Union and emigrated in 1989. They arrived in Moscow in mid-February to attend a wedding.
(Reuters, ITAR-TASS)