United States President Barack Obama has warned North Korea that it will "achieve nothing by threats or by provocations".
The warning comes as Pyongyang prepares to launch a long-range missile which it says will put a satellite in orbit.
Mr Obama was speaking after talks in Seoul with South Korean President Lee Myung-bak on Sunday, on the eve of a two-day global summit on nuclear security, the BBC reports.
The leaders said North Korea risked further sanctions and isolation if it did not cancel its launch plans.
Mr Obama said Washington and Seoul were "absolutely united" that "bad behaviour" by North Korea would not be rewarded.
"North Korea knows its obligations and it must take irreversible steps to meet those obligations."
The launch will contravene an agreement Pyongyang reached in February which would have seen it receive food aid in exchange for a partial freeze on nuclear activities and an end to ballistics tests.
Mr Lee said the countries had "agreed to respond sternly to any provocations and threats by the North and to continually enhance the firm South Korea-US defence readiness".
Howeber, he said the international community was ready to help North Korea improve the lives of its citizens if it chose a path of peace.
Barack Obama also criticised China, saying its refusal to challenge North Korea on the nuclear issue was not working as a policy.
The president also visited the demilitarised zone between the Koreas and spoke to US troops at a camp near the frontline and then walked into the heavily patrolled area.