South Korea's President Roh Moo-hyun is in Washington for talks with U.S. officials and lawmakers. Differences over how to persuade North Korea to abandon its nuclear program are expected to dominate discussions.

South Korea's President Roh Moo-hyun (r) with US Secretary of State Condoleezza Rice in Washington

In recent years, the main source of tension in relations between the United States and South Korea has been differing views on how to deal with North Korea.

Don Oberdorfer, of Johns Hopkins University's School of Advanced International Studies, says unity against North Korea is "the heart" of the U.S.-South Korean military alliance. But, he says South Korean President Roh, in his efforts to persuade Pyongyang to give up nuclear weapons, has tried to be friendlier to North Korea while the Bush administration has pushed an increasingly harder line.

"In recent months, especially since last September, the United States has moved toward simply applying additional pressure, financial pressure and other pressures, to North Korea," said Oberdorfer. "It's moved away, in effect, from some of the ideas about negotiation."

South Korea has indicated that it would like to see what it calls "more flexibility" in the ongoing six-party talks over the North Korean nuclear program. The talks include North Korea, South Korea, China, Japan, Russia and the United States.

Oberdorfer says the issue will certainly be discussed during President Roh's visit to the White House. Although he does not expect any major developments to be announced from this meeting, he said it will give the two leaders an opportunity to work on improving personal relations.

"The two guys' relationship is very much up in the air and it's very uncertain," he said. "Their meeting last November was not very good, and the hopes on both sides is that there will be a better chemistry and a better understanding between the leaders of the United States and the Republic of Korea [South Korea]."

The importance of personal relations between President Bush and President Roh was also pointed to by James Auer, a retired naval commander who is now director of Vanderbilt University's Center for Japanese Studies.

He says since the United States is South Korea's only military ally, it is "unthinkable" for President Roh not to try to strengthen relations with the United States.

"And he's got to put on the appearance that he does value and is maintaining the relationship with the United States," said Auer. "And, again, he will be cordially received, correctly received, but I don't think it's going to be warm and fuzzy. You don't see the president offering to bring him to Crawford, or taking him to Graceland, the way the president did recently with [Prime Minister] Koizumi from Japan."

A senior U.S. administration official, who spoke on background, downplayed any disputes. He acknowledged differences of opinions between Seoul and Washington on the best way to negotiate with North Korea. But he said the two sides have to "figure out what works best" - not only for them, but also for the other countries in the six-party talks.

He said both sides had never planned on producing a joint statement during this visit, and he urged journalists not to attach any significance to it.

Meanwhile, on a separate issue, the U.S. official said no decisions have been made about handing over wartime command control, from the U.S. to the South Korean military. He did emphasize, though, that even as the U.S.-South Korean alliance continues to evolve, the United States security commitment to South Korea will remain "rock solid."

President Roh meets with President Bush at the White House Thursday. He will also stop in San Francisco, before leaving the United States on Friday.