To make it clear, here is the paragraph I noticed:

Under the deal, South Korea is allowed to build ballistic missiles with a range of up to 497 miles but is barred from tipping them with warheads weighing more than 500 kilograms, or half a ton, because of concerns about a regional arms race. South Korea wants to double the upper limit of the payload to a ton, officials here said. The South has no nuclear weapons of its own, and decades ago the United States intervened to stop a secret program to build them.

Emphasis mine.