A frosty quiet hung over the broad Yalu river that divides North Korea from China on Tuesday, after Pyongyang ruptured the diplomatic atmosphere with its third atomic weapons test.

More Chinese troops than usual -- six rather than the standard two or three -- stood guard on the Sino-Korean Friendship Bridge linking the two countries.

Beijing's reaction to Pyongyang's atomic blast is seen as key. It is the North's sole major ally and biggest trading partner, with most of the business passing through Dandong in China's far northeast.

China's foreign ministry expressed "firm opposition" to the test, saying the North had gone ahead"despite widespread opposition from the international community". But it made no immediate threat of sanctions or reprisals.

The crossing was quiet Tuesday, as it has been in recent days with both countries marking the lunar new year.

No signs of jubilation about the North's test were visible on its side of the river, where guardhouses line the bank every 200 metres or so. Uniformed soldiers were conspicuous by their absence.

Normally soldiers stand outside each building. But this time they kept out of sight as tourist boats skimmed along the river, only emerging once the vessels were well downstream.

The snow-filled streets of the North Korean town of Sinuiju across the bridge were almost deserted.

"I feel the test is dangerous for China and Dandong," said one resident of Dandong, declining to give his name. "I think the test will influence China-North Korea relations and will affect trading."

Further upriver, several kilometres from Dandong itself, North Korean soldiers could be seen on the banks.

But they shied away from a Chinese tourist boat, not even taking a few steps towards the water to pick up a tossed packet of cigarettes, as if they were on alert.

One soldier carrying an assault rifle pointed his finger at a boat, yelling.

Closer to the test site, at the Rujia Modern Hotel in Erdaobaihe in Jilin province which also borders North Korea, a woman said that at the time of the blast: "I heard people shouting, 'Earthquake! Earthquake!'"

No buildings collapsed and no one was injured, she added on condition of anonymity. Another woman in the town said she felt shaking for less than a minute.

On the streets of Beijing, views were mixed.

One man who gave his name as Li said: "China shouldn't bully North Korea. China should treat North Korea as a good friend. As a powerful nation and people, a country must conduct nuclear tests."

But Gerry Deng was more ambivalent. "I want the world to be peaceful," she said. "I don't think this country (North Korea) has the right to conduct tests, but I also don't think other countries have the right to interfere."