WASHINGTON (AFP) - In the long diplomatic struggle over North Korea's nuclear program, sanctions have rarely gotten a rise out of the regime in Pyongyang, which time and again has proved impervious to outside pressure.

Except once.

That was in September 2005 when the United States unilaterally blacklisted a small family-owned bank in Macau, Banco Delta Asia (BDA), with just 25 million US dollars in North Korean funds.

That seemingly small move, experts say, set off a chain reaction as other banks in Chinese territory scrambled to cut off their ties to North Korea to avoid similar action, bringing Beijing into play and exposing a weakness in Pyongyang's armour.

That might be the pressure point that the United States is looking for as it considers a new round of sanctions against North Korea in the wake of its nuclear test on Sunday.

North Korea has "pretty weak and brittle connections to the international financial system. They don't have a lot of money," said Marcus Noland, an expert at the Peterson Institute for International Economics.

"As we saw in the BDA case, it doesn't take a whole lot of disruption to seemingly generate real effects within North Korea," he told AFP.

US Ambassador to the United Nations, Susan Rice, vowed Tuesday that North Korea will "pay a price for the path that they're on if they don't reverse."

But as the United States seeks to mobilize international support for tougher sanctions, the record over the last 15 years also suggests that a new round of sanctions is unlikely to make a lasting impression.

"Sanctions are almost never successful in achieving the objectives that they set out to achieve, other than to make sure that something that happens that people don't care for, can't tolerate, gets a response," said Douglas Paal, of the Carnegie Endowment for International Peace.

"Globally, sanctions have not worked particularly well except in the case of South Africa where there was a consensus not only by the broad civilized world, but also by South Africa's neighbors that they would support the efforts," he said.

After North Korea's first nuclear test in October 2006, the UN Security Council passed a resolution banning trade in luxury goods, military weapons and nuclear technology with North Korea.

Experts say the 2006 resolution is likely to be the foundation for any new round of sanctions, which could be expanded to include a freeze on North Korean assets abroad and denial of access to the international banking and financial services.

Another tact is likely to be to strengthen cooperation on interdicting North Korean military or nuclear-related shipments. South Korea, for instance, has said it will join the US-led Proliferation Security Initiative.

But a key question is whether the sanctions will be enforced by China, North Korea's most important trading partner and its main contact with the outside world.

"China remains the screen door through which a lot flows to North Korea," said Paal.

"The records show they have not implemented the initial set of sanctions on luxury goods. There may be episodic or emblematic efforts to stop things, but it's not a sustained imposition of sanctions," he said.

That record might lead the United States to seek approaches that bypass the UN Security Council, as it did in the BDA case in Macau, experts said.

"Implementing sanctions is always difficult for any government because the nature of sanctions is that you are asking your citizens to not do something they want to do," said Noland. "So you immediately create a domestic political constituency against the policy."

"What happened in the BDA case was very interesting because it turned this political logic on its head," he said.

Chinese banks with little if any involvement with North Korea suddenly found themselves facing the prospect of being frozen out of the US market, he said.

"So their incentive was not to oppose sanctions, their incentive was to encourage their government to implement them enthusiastically in order to give those banks a clean bill of health so they could continue to operate in the United States," he said.

Although analysts were pessimistic that even better enforced sanctions will do much good, they said they still should be tried.

"If we have to play for time, it's still important to be firm, it is still important to maintain a united front with the other parties concerned," said Richard Bush, a former State Department official.

"And it's necessary to ratchet up the pressure at least a little bit, to show there are consequences to these provocative actions," he told AFP.