But Beijing will likely never risk a scenario that could prompt waves of North Koreans to flee across the border and at the same time leave nuclear material unsupervised.
"China is coming to the conclusion that North Korea is becoming a liability and it needs to take steps to deal with it," said Paul Haenle, former China director on the U.S. National Security Council and White House representative to stalled six-party talks aimed at getting Pyongyang to end its nuclear activities. "(But) I don't think we'll see dramatic shifts overnight and I don't think we'll see publicly announced shifts. This is the kind of thing that will happen gradually and will happen behind the scenes."
Over the weekend, China's foreign minister said it would not allow "trouble making" on its doorstep while President Xi Jinping appeared to rebuke North Korea during a speech in which he said no country should be allowed to cause chaos "for selfish gain."
(Read More: China (Indirectly) Just Told North Korea to Knock It Off)
U.S. lawmakers in turn criticized China, saying it had failed to rein in North Korea.
But Beijing does not have the influence it is often assumed to have in North Korea, a country used to isolation and deeply suspicious of outsiders, even of China, which came to Pyongyang's aid in the 1950-53 Korean War.
"North Korea and China do not depend on each other as much as some people think," said Chung Young-chul, a North Korea expert at Sogang University's Graduate School of Public Policy in Seoul.
"Culturally and historically, as South Koreans do, North Koreans also have a penchant for demeaning the Chinese ... And North Koreans strongly oppose being flunkies for China."
Sanction Implementation Hard to Gauge
China negotiated the U.N. sanctions with the U.S. that were imposed in response to the Feb. 12 nuclear test and has said it wanted them implemented.
The measures tighten financial curbs on North Korea, order checks of suspicious cargo and strengthen a ban on luxury goods entering the country.
China has said nothing publicly about what it has done to implement the sanctions on the ground.
Some commodities traders have noted a slowdown in the trade of iron ore and coking coal from North Korea. South Korean media has said China has been refusing to renew visas for North Korean factory workers in the Chinese border city of Dandong.
(Read More: North Korea and China Get Cozier on Trade: Expert)
One South Korean media report also said China had fired a warning shot to North Korean banks in China, telling them to stay within the remit of their permitted operations.
China did not export crude oil to North Korea in February, customs data showed, the first absence of deliveries since the same month in 2012. But data has also shown no exports in February 2011. Data for March has not been released yet.
In any case, the question remains: how to punish a country which has so few links with the outside world?
"For an economically closed country whose government does not make the people's welfare its mission, the effects of sanctions will not be obvious," the Economic Observer, a well-regarded weekly Chinese business newspaper, wrote in a commentary on its website on Monday.
"It's like trying to tell a thin person to watch their weight."
This isn't to say that Beijing's hands are totally tied.
Having backed the latest U.N. sanctions, China could enforce them to exert just enough pain to get North Korea back to the negotiating table but not enough to cause the country to implode, the widely read and influential tabloid the Global Times said on Monday.
"This pressure should not cause a horrible worsening of the domestic situation in North Korea but let them know how important outside aid is to the country," wrote Yang Lei, an international relations professor at Tianjin's Nankai University.
Beijing would never do anything as drastic as cutting trade ties, as some U.S. politicians have demanded, because of the impact it would have on China, said Sun Zhe, a China-U.S. expert at Beijing's elite Tsinghua University.
To ease tensions, Washington should end the military exercises with South Korea that have riled the North and offer to hold talks with Pyongyang, he added.
"You are asking China to do something very serious, and yet the U.S. government won't make even a symbolic move like stopping military drills," Sun said.