US President Barack Obama telephoned Russian President Dmitry Medvedev on Monday to offer condolences following the suicide attacks on Moscow's metro system that left at least 38 people dead.
Obama pledged US cooperation in assisting the Russian government track down those responsible for the attacks and bring them to justice, the White House said.
Authorities believe two female suicide bombers carried out separate morning attacks on the metro system in what was the worst terrorist attack on the Russian capital in six years.
Obama had earlier issued a statement condemning the attacks and expressed solidarity with the Russian people.
"The American people stand united with the people of Russia in opposition to violent extremism and heinous terrorist attacks that demonstrate such disregard for human life, and we condemn these outrageous acts," Obama said in a statement released by the White House.
"My thoughts and prayers go out to those who lost loved ones, and I wish all who sustained injuries a successful recovery," he said.
Meanwhile, US authorities were stepping up security in the subway stations in Washington and New York. Metro authorities in Washington announced there would be security sweeps throughout the day.