North Korean leader Kim Jong Un celebrated his birthday - supposedly his 33rd - early by testing a hydrogen nuclear device, sending seismic waves into the earth and shockwaves across the globe.

The move drew international condemnation and vows of far tougher sanctions from the United Nations Security Council as well as the United States, which is considering deploying strategic military assets to South Korea, its ally.

Would such measures work on the recalcitrant North? After all, it risked burning its bridges by not informing its closest ally China beforehand about Wednesday's test. And new sanctions would likely negate promises of economic development and a better life that Mr Kim made to his countrymen in his New Year speech.

In retaliation, Seoul is set to resume loudspeaker propaganda broadcasts along the border with Pyongyang today, a move certain to annoy Mr Kim.

When the South deployed the loudspeakers last August to retaliate against a landmine attack blamed on the North, Mr Kim threatened war but ended up initiating talks and striking a deal with Seoul.

It remains to be seen how long the latest round of escalated tensions will last, but inter-Korea relations are definitely frozen for now.

Some observers say Pyongyang's aim is to press Washington into resuming dialogue, but it is doubtful whether the Obama administration will be willing to enter into negotiations during the last leg of its term.

Unless the international community is ready to come to a truce with the Kim regime, "denuclearising North Korea at this moment is almost impossible", said international relations professor Park Ihn Hwi of Ewha Womans University.