He pledged Washington's full support for the ongoing search-and-rescue efforts, stressing South Korea is one of America's closest friends.
"On behalf of all the American people, Michelle and I send our deepest and heartfelt condolences to the families of the victims of the tragic ferry sinking off the coast of the Republic of Korea,"
Obama said in a statement. "Our hearts ache to see our Korean friends going through such a terrible loss, especially the loss of so many young students."
The 6,325-ton South Korean ferry, the Sewol, sank off the nation's southwestern coast Wednesday morning (local time).
It was carrying 475 passengers and crew members, traveling from Incheon, west of Seoul, toward the Jeju Island. The passengers include 325 students and 15 teachers from Danwon High School in Ansan, just south of Seoul. The students were on a four-day field trip.
Twenty five of those aboard were found dead and 271 others remain unaccounted for. The other 179 have been rescued.
Obama added he has directed the U.S. military to provide every assistance requested by South Korea in the days ahead.
He cited his plan to visit South Korea next week, saying he would use it to demonstrate Washington's unwavering commitment to its ally.
Obama is scheduled to arrive in Seoul on Friday for an overnight stay after traveling to Tokyo.
USS Bonhomme Richard, an amphibious assault ship carrying MH-60 Seahawk helicopters, has been already assisting the search.
A Pentagon official said there is no specific decision yet on sending another vessel.
The British government joined the wave of condolences and vowed any support needed.
"I send my deepest condolences to the government and people of South Korea following the Sewol ferry disaster. My thoughts are with all the bereaved families and their relatives and friends, and those people still waiting for news of their loved ones," Foreign Secretary William Hague said in a statement. "The British government is ready to offer any assistance we can to the Korean government at this tragic time."
U.N. Secretary-General Ban Ki-moon earlier expressed his sympathy to the families of victims and other people in his homeland. (Yonhap)