By Penny Spiller
Members of the UN Security Council were unanimous in their call for sanctions against North Korea following its nuclear test last week.
North Korean ships will be subjected to more searches
But now, as countries consider the details of these sanctions, that unity is beginning to look skin deep.
One of US Secretary of State Condoleezza Rice's toughest tasks while in Asia will be to persuade South Korea and China to step up efforts to stop and search cargo ships going in and out of the North, as allowed for under the UN resolution.
In South Korea, Ms Rice will also be asking Seoul to expand its role in the US-led Proliferation Security Initiative (PSI). This was launched in 2003 as a way of stopping the spread of weapons of mass destruction by encouraging countries to carry out searches of suspicious vessels in their territorial waters.
In the wake of the nuclear test, the US is very concerned that North Korea's impoverished government may be tempted to sell nuclear technology to other states or organisations.
But South Korea has been reluctant to be anything other than an observer to the PSI, for fear of antagonising its northern neighbour and compromising its policy of engagement with Pyongyang.
It also shares the same fear as China that any attempt to forcibly board a North Korean ship at sea could spark a military clash that may even lead to war.
But Seoul's participation in the initiative is seen as critical, because many ships to the North pass through its waters.
One South Korean opposition politician, Song Young-sun, recently said she believed North Korea had moved nuclear materials for its recent test through South Korean waters.
LAW OF THE SEA
All ships have a right to free passage in international waters
They also have the right of "innocent passage" through a country's territorial seas
Interdiction is the process of stopping and inspecting ships
Country has a right to interdict ship outside its territorial seas if seen as a threat
Boarding a ship at sea is difficult and can quickly become confrontational
She said a number of the ships that sailed through South Korea's Jeju Strait went from a port in the west of North Korea, to Kimchaek port in the east, not far from where the nuclear test was carried out.
China - the nearest country the North has to an ally, and a vital supplier of financial, energy and food assistance to the impoverished nation - has also voiced concerns over ship searches.
It voted for sanctions against North Korea only once the wording on ship checks was changed.
It has begun inspecting cargo crossing its land border, but has opposed the interdiction of ships.
Analysts say that, as well as the fear of an armed confrontation with North Korea, China is concerned the US may abuse its naval power if the searches are allowed to go ahead.
Philip Roche, a partner at international law firm Norton Rose, says the stop and search of North Korean ships is a way of effectively punishing and ostracising Pyongyang.
"It's one thing to interdict a ship because you suspect it of carrying materials that are a threat to your country, but the sanctions would allow for the inspection of a ship carrying pretty much anything except food and aid," he said.
And he said China and South Korea were right to be concerned about a confrontation.
"Effectively, a North Korean ship is in law a little piece of North Korea on the seas. Going on board a ship, particularly using armed force, would to them be tantamount to an act of aggression," he said.
On her way to Asia, Ms Rice said she believed all states were "determined to carry out" a set of obligations under the sanctions, which included the inspection of "certain cargoes".
Both China and South Korea have taken a stand against the North's recent actions, but it is not yet clear if that will stretch to searches at sea.