A senior intelligence official on Thursday defended U.S. efforts to monitor an alleged Syrian nuclear facility that the U.S. says was built with North Korean help.

Joseph DeTrani, the national intelligence director's mission manager for North Korea, said U.S. intelligence had been watching the reactor very closely and for a number of years.

"This was not a failure," he told an audience at the Center for Strategic and International Studies think tank. "Action was taken when it was going operational, and they are now out of business."

The structure in the remote eastern desert of Syria was bombed by Israeli jets in September 2007. Syria's ambassador to the United States has said the United States fabricated pictures allegedly taken inside.

Senior U.S. intelligence officials have said they believe it was a secret nuclear reactor meant to produce plutonium, which can be used to make high-yield nuclear weapons. They alleged that North Korea aided in the design, construction and outfitting of the building.

DeTrani's comments came as Christopher Hill, the top U.S. envoy to nuclear talks with North Korea, conceded that the North's disarmament will be difficult to achieve this year.

Disarmament talks stalled when the North missed a deadline at the end of last year to give a full accounting of its nuclear programs.

The United States previously insisted that any declaration include North Korea's alleged uranium enrichment program and nuclear cooperation with Syria. The U.S. apparently has backed down from those demands, drawing strong criticism.

The United States and North Korea have been trying to hash out a timeline for the declaration, along with American concessions that include removing Pyongyang from terrorism and economic sanctions blacklists.

DeTrani said the key to the talks is "verification, verification, verification." The United States, he said, would insist that, once the North handed over a "complete and correct" declaration, it would then have to dismantle all its nuclear programs and weapons.

As a result of six-nation nuclear talks, the North has stopped making plutonium and begun disabling its nuclear facilities so they cannot be quickly restarted. It still has a stockpile of radioactive material that experts believe is enough to make about a half-dozen bombs.