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.
North Korea kept up its harsh rhetoric Friday, indicating that it saw upcoming military exercises as fresh provocation. US Gen. Walter Sharp toured the island area attacked by the North on Tuesday.