US President George W Bush has called South Korea's Lee Myung-bak to hail his election victory and discuss future de-nuclearisation in North Korea.
Lee made his first policy speech the morning after victory
The White House said the two leaders talked of their dedication to a Korean peninsula free of nuclear weapons.
And Mr Bush invited president-elect Mr Lee to the US after his inauguration, which is expected in February.
Mr Lee has pledged to forge closer ties with Washington, speaking of a lack of trust under incumbent Roh Moo-hyun.
He also said relations with Japan must be improved, as should the overall relationship between nations in north-east Asia.
Japanese Prime Minister Yasuo Fukuda congratulated Mr Lee, saying he hoped for "further development in amicable, co-operative relations".
White House spokesman Gordon Johndroe said Mr Bush told Mr Lee he looked forward to a relationship based on "mutual trust and understanding, to achieve peace and prosperity".
"The two leaders discussed their dedication to a nuclear weapons-free Korean peninsula and the need to continue to show North Korea there is a better way forward if they live up to their commitments," said Mr Johndroe.
Earlier, Mr Lee signalled a tougher line towards Pyongyang, saying the North must scrap its nuclear ambitions in order to gain full economic co-operation with Seoul.
"Full-fledged economic exchanges can start after North Korea dismantles its nuclear weapons."
He also called for the North to improve its human rights record.
Leaders of the two Koreas held a landmark summit in October
"Previous governments have refrained from criticising and tried to unilaterally appease the North Koreans. I can say such a thing will change," he said.
Full results from Wednesday's election show Mr Lee, a conservative, won a landslide victory.
He took 48.7% of the vote, putting him 22 percentage points ahead of nearest rival Chung Dong-young.
The victory hands back power to Mr Lee's Grand National Party (GNP) for the first time in a decade - ending the domination of President Roh's liberal United New Democratic Party (UNDP).
Over the past decade Seoul has followed a policy of engagement with the North and tensions have eased.
The two countries remain technically at war as they have never signed a peace deal in the years since the Korean War of 1950-53.
In October opposing leaders met for only the second time and signed a wide-ranging accord calling for peace and improved economic ties.
Analysts say that as long as North Korea continues moves to disarm, Mr Lee's stance should not dramatically affect ties between Seoul and Pyongyang.
The president-elect also addressed the economy - an issue that dominated campaigning and was where the former Hyundai executive struck a chord with voters.
He said his government would court foreign investment and would "foster an environment where companies can operate freely".
Mr Lee will replace the incumbent, Mr Roh, when his five-year term expires in February.
But the president-elect still faces claims he was involved in a fraud in 2001 - allegations he denies.
Although prosecutors had decided there was not enough evidence to charge him, MPs voted to reopen the case earlier this week when new evidence appeared.
A parliamentary probe is expected to be completed before Mr Lee takes office.
But correspondents say that for many voters, how the president-elect handles the economy is more important than the allegations against him.