On Monday, State Department spokesman P.J. Crowley told reporters the administration is studying the evidence a group of visiting American scientists used to conclude the North was building the enrichment facility, which presumably could be used to produce fuel for nuclear weapons.
"We will not be drawn into rewarding North Korea for bad behavior," he said. "They frequently anticipate doing something outrageous or provocative and forcing us to jump through hoops as a result. We're not going to buy into this cycle."
The North's artillery on Tuesday struck the small South Korean-held island of Yeonpyeong, which houses military installations and a small civilian population and which has been the focus of two previous deadly battles between the Koreas.
After the North's barrages, South Korea responded by firing self-propelled howitzers, but an official at South Korea's Joint Chiefs of Staff declined to say whether North Korean territory was hit.
Earlier this month, during a speech to U.S. troops stationed in South Korea, President Barack Obama said: "Pyongyang should not be mistaken: The United States will never waver in our commitment to the security of the Republic of Korea. We will not waver."