A South Korean man has been hospitalized with wounds from an explosion during the Boston Marathon, which left three dead and more than 170 others hurt, officials said Tuesday.

Ahn Dong-sik, 23, underwent minor surgery at a nearby hospital for injuries on his legs and thighs, but he is not in critical condition, according to officials at the South Korean consulate general in Boston.

Ahn, in Boston for a language study program, briefly lost consciousness in the stands near the marathon's finish line when the bombs went off.

"I am okay," he said in a smartphone message. "My wound is not a big problem. I saw many people hurt right in front of my eyes."

Monday's two blasts in Boston marked the first major bombing attack in the U.S. since Sept. 11, 2001.

The Boston case provided a reminder of continued challenges facing the U.S. from terrorist attacks. No individual or group has claimed responsibility yet.

Speaking to reporters at the White House, President Barack Obama branded it as a "cowardly" act of terror.

"This was a heinous and cowardly act," he said. "Any time bombs are used to target innocent civilians, it is an act of terror."

 Earlier in the day, Obama issued a proclamation ordering that the U.S. flag be flown at half staff at the White House and at all public buildings and grounds, as well as military posts.

In Seoul, South Korean President Park Geun-hye offered condolences to the U.S., saying she can't contain her shock at the news. (Yonhap News)