South Korea's statement that it intends this year to begin preparations to reunify the Koreas will likely irk China and the North, which could consider it a provocation.
South Korea launched a planned round of multi-day military exercises Wednesday, while rebuffing the North's recent conciliatory gestures.
North Korea sees South Korea's plans to conduct a live-fire artillery drill this weekend as "needlessly provocative," but the US insists it is normal and necessary.
The US was hoping that China, the only country with diplomatic influence over North Korea, would rebuke the country for shelling South Korea last month. But China appears intent on maintaining support for Kim Jong Il.
In a phone call Monday, President Obama asked Chinese President Hu Jintao to put pressure on North Korea, even as US held military drills with South Korea.