The United States reaffirmed its strong alliance with South Korea on Thursday and said it won't be "deterred by senseless acts of violence" despite the bloody attack on its ambassador to Seoul.

State Department deputy spokeswoman Marie Harf also said in a statement that Amb. Mark Lippert is recovering in a hospital after surgery, and "is doing well and in great spirits." She cited a message the ambassador tweeted earlier in the day.

"The U.S.-ROK alliance is strong; we will not be deterred by senseless acts of violence," she said.

Harf also noted that both South Korean President Park Geun-hye and Foreign Minister Yun Byung-se called Lippert to express condolences and wish him a speedy recovery.

"Ambassador Lippert looks forward to getting back to work with his Republic of Korean counterparts to strengthen our bilateral relationship and to jointly address regional and global challenges," the spokeswoman said.

Harf said the suspect is in custody and U.S. law enforcement is cooperating closely with Korean police in the investigation. She also said she "cannot speculate on a motive at this time."

The attack -- the first terrorist attack on a top U.S. envoy in Seoul -- occurred at a breakfast function in central Seoul, where Lippert planned to give a lecture organized by a coalition of South Korean civic groups dedicated to forge national consensus on unification with North Korea.

The knife-wielding assailant, identified as 55-year-old Kim Ki-jong, slashed Lippert on the face and wrist when he was having breakfast. The ambassador was immediately taken to a hospital and received more than 80 stitches. Hospital officials said he will need to remain hospitalized for three to four days.

The attacker was immediately arrested. As he was hauled away, Kim shouted his opposition to joint military exercises under way between South Korea and the U.S. North Korea has strongly condemned such annual drills as a rehearsal for an invasion of the country.

In Saudi Arabia, Secretary of State John Kerry also said the U.S. "will never be intimidated or deterred by threat or by anybody who harms any American diplomat."

"We will remain as resolved as ever to pursue what we believe is in the interests of our country and with respect to universal rights and values," Kerry told reporters during a visit to the Middle Eastern nation, according to the department.

"And whoever threatens or harms our diplomats, I can assure them, will be prosecuted to the fullest extent of the law," he said.

Rep. Ed Royce (R-CA), chairman of the House Foreign Affairs Committee, also called the alliance between the U.S. and South Korea "very strong, and very important."

"I was very concerned to learn of the violent attack on Ambassador Lippert in Seoul. My thoughts are with the Ambassador as he begins his recovery from the injuries sustained in this attack," Royce said in a statement.

"Ambassador Lippert holds an important post, entrusted with the very strong, and very important, relationship between the United States and South Korea. I am thankful to Ambassador Lippert and the many U.S. diplomats abroad. They do important work," he said.

U.S. experts said the incident won't have any impact on relations between the two countries.

"This kind of an attack can happen in any country, including the United States, and by people from any country. The American people and American officials understand this, so it will have absolutely no effect on U.S.-Korean relations," said David Straub, associate director for the Korean Studies Program at Stanford University.

Such a case is "a one- or two-day story in the media" in the U.S., he added.

Evans Revere, a former principal deputy assistant secretary who also served as duty chief of mission at the U.S. Embassy in Seoul, said the alliance-partnership between the two countries is stronger than ever, certainly strong enough and resilient enough to deal with this reprehensible act."

Lippert's father expressed shock and worries.

"This was something we felt pretty secure in because South Korea is a friendly country. He seemed to be well received, and seemed to be progressing really well within the citizenry," the ambassador's father, Jim Lippert, said at his home in Ohio, according to WCPO television.

"So it did come as a shock to us that in that venue he would be subjected to an assault of this nature," he said. He added the ambassador never got hurt when he served in Afghanistan and Iraq as an intelligence officer with the Navy SEALS.

The father said he is proud of his son.

"How does it make me feel? I'm sorrowful that it happened. It caused me great anxiety. It caused my family great anxiety and despair," he said. "But how do I feel about him? I feel proud of him." (Yonhap)