The chief US envoy at North Korean nuclear talks told lawmakers Wednesday that the United States would be persistent in finding out how far North Korea had progressed on a secret uranium-based weapons program. Lawmakers are pressing US negotiators to make sure any such program is accounted for as the North declares its nuclear efforts as part of a Feb. 13 six-nation disarmament agreement. Assistant Secretary of State Christopher Hill said the United States knows that North Korea has bought equipment that could be used only for uranium enrichment, but he expressed uncertainty about the program's current state. "How far they've gotten; whether they've actually been able to produce highly enriched uranium at this time - I mean these are issues that intelligence analysts grapple with. But what we know is they have made the purchases, and we need to have complete clarity on this program," Hill told a House of Representatives' Foreign Affairs Committee hearing.