From the New York Post :

July 9, 2006 — SEOUL, South Korea – A program of covert action against nuclear and missile traffic to North Korea and Iran is to be intensified after last week’s missile tests by the North Korean regime.
Intelligence agencies, navies and air forces from at least 13 nations are quietly co-operating in a “secret war” against the outlaw regime.
It has so far involved interceptions of North Korean ships at sea, multinational naval and air surveillance missions out of Singapore, investigators poring over the books of dubious banks in the former Portuguese colony of Macau and a fleet of planes and ships eavesdropping on the “hermit kingdom.”
Few details filter out from Western officials about the program, which has operated since 2003. But it has tightened a noose around Kim Jong Il’s bankrupt nation.
“Diplomacy alone has not worked, military action is not on the table and so you’ll see a persistent increase in this kind of pressure,” said an official.
In a telling example of the program’s success, two Bush-administration officials indicated last year that it had blocked North Korea from obtaining equipment used to make missile propellant.
The Americans also persuaded China to stop the sale of chemicals for North Korea’s nuclear weapons scientists. And a shipload of “precursor chemicals” for weapons was seized in Taiwan before it could reach a North Korean port.
According to John Bolton, the U.S. ambassador to the United Nations and the man who originally devised the program, it has made a serious dent in North Korea’s revenues from ballistic missile sales.