The United States' chief negotiator on North Korea's nuclear programs has said Pyongyang needs to be open about its enriched uranium activities in order for an agreement on denuclearization to move forward. But, as Daniel Schearf reports from Beijing, the issue could prove a sticking point, as North Korea has never publicly admitted to having such a program.

The U.S. envoy to talks on North Korea's nuclear programs, Christopher Hill, told reporters Friday he aims to seek an explanation from North Korea on its uranium enrichment program.

In an agreement reached in February the North would receive aid and improved diplomatic ties if it declared its nuclear facilities and shut down its main Yongbyon nuclear reactor.

Hill said this means that North Korea must provide an explanation of the alleged uranium enrichment program.

"This is an issue that must be addressed and must be resolved because we can't have a complete declaration unless there's been a complete understanding of the highly enriched uranium issue," he said.

The U.S. first accused Pyongyang of having a secret highly enriched uranium program in 2002. That prompted North Korea to kick nuclear inspectors out of the country, withdraw from the nuclear non-proliferation treaty and restart its plutonium reactor.

That reactor provided the nuclear material for North Korea's first successful nuclear test in October. The U.S. fears Pyongyang may be hiding a highly enriched uranium program, although the North denies it.

Hill said he would be discussing the alleged enriched uranium program with North Korea's chief negotiator Kim Kye-Kwan when they meet on Saturday in Beijing for a working group on denuclearizing the North.

Hill said they would also discuss the U.S. Treasury's decision this week to ban U.S. banks from doing business with a Macau bank, Banco Delta Asia. Washington says this bank helped Pyongyang launder money and deal in counterfeit currency and cigarettes.

Banco Delta Asia's chairman, Stanley Au, on Friday denied intentionally helping Pyongyang with any illegal activity.

South Korea said Friday it would resume shipments of fertilizer aid to North Korea later this month, after a gap of almost a year in response to a North Korean missile test.

The various steps - in Washington, South Korea and Beijing - are all a prelude to the sixth round of six-nation talks to begin Monday in Beijing, which include North Korea, Japan, and Russia.