"The launch pad appears to be operational,'' Brown said.
North Korea last launched a long-range missile in April. It has since carried out a nuclear test and several short-range missile tests.
The news of the suspected launch preparation came as South Korean media reported on Friday of US intentions to implement financial sanctions on Pyongyang in an effort to punish the country over its weapons trading and counterfeit activities.
Warning the North that its actions will "no longer be rewarded", James Steinberg, the US deputy secretary of state, told Lee Myung-bak, the South Korean president, that "North Korea would be mistaken if it thinks it can make provocations and then get what it wants through negotiation as it did in the past. The US won't repeat the same mistake again".
Steinberg, on a four-day visit to Seoul, is accompanied by a senior US treasury official who oversaw previous financial restrictions on the North.
As international pressure heightens, tensions continue to rise on the Korean peninsula, and on Thursday, South Korean officials said a patrol boat from the North entered its waters around their disputed maritime border, but backed off after nearly an hour following repeated warnings.
The naval standoff came amid concerns that the North might try to provoke an armed clash in the area - the scene of deadly naval skirmishes in 1999 and 2002.
Meanwhile, international powers continue to work on an appropriate response to the North's nuclear test and missile launches.
On Thursday. ambassadors from key nations say they are nearing an agreement on new UN-backed sanctions against North Korea for violating UN resolutions and conducting a second nuclear test.
The proposals for a new UN resolution are being discussed by the five veto-wielding Security Council nations - the US, China, Russia, Britain and France - and also Japan and South Korea, the two countries most closely affected by the test.
They have been holding closed-door meetings since May 26, a day after North Korea's underground nuclear test.