A recent report by US Senate Republican staff members warns that China, because of its deepening economic ties with North Korea as well as its ancient claims on Korean land, could attempt to "manage, and conceivably block" an eventual unification between the two Koreas, if ever the Kim family falls from power in Pyongyang.

The report was released last month with little fanfare, but North Korea watchers say it gives voice to an increasingly popular but still-sensitive sentiment: that China will ultimately try to prevent the South from absorbing the North, the long-assumed post-collapse scenario.

Such a situation is well down the road, experts say, but it resonates at a time when China is being aggressive elsewhere in the region, staking claim to much of the South China Sea and to islands administered by Japan.

China might act with similar aggression in North Korea, the report argues, to "safeguard its own commercial assets, and to assert its right to preserve the northern part of the peninsula within China's sphere of influence".

The report was written primarily by Keith Luse, an east Asia specialist who worked as an aide for the recently defeated Republican senator Dick Lugar, who had been a member of the Senate committee on foreign relations with a long-standing interest in North Korea. The minority staff report, Luse said in an email, was written to inform committee members – including John Kerry, nominated by Barack Obama as the next secretary of state – "to not expect an East-West Germany repeat situation" regarding unification between the Koreas.

A Kerry spokesman said neither Kerry nor his staff would comment on the report, adding that the senator has declined all interviews since his nomination.

The tight connection between China and North Korea represents a major policy challenge both for the Obama administration and for the incoming government in South Korea. Conceivably, both Washington and Seoul could try to try to re-engage with Pyongyang, but neither finds that palatable. Washington failed to influence North Korea's behaviour during previous periods of one-on-one and multinational talks. South Korean president-elect Park Geun-hye, a conservative, says she won't reinstate major projects with the North unless the family-run police state dismantles its nuclear weapons. The North says it never will.

Outside analysts see no clear sign of instability in North Korea, under third-generation leader Kim Jong-un. But the report lays out how China might respond if North Korea is teetering or collapsing. China could send its own troops into North Korea to prevent a mass exodus of refugees, the report says, citing conversations between Chinese officials and Senate staff. China might also try to use a protracted UN process to determine which nation – China or South Korea – has legitimate authority over the North.

"Anybody who is a serious analyst can't discount this as a plausible scenario," said Victor Cha, the Korea chair at the Centre for Strategic and International Studies, referring to the general argument of the report.

From a US perspective, Cha said, the greatest concern is how poorly prepared other countries are to deal with – and co-operate during – a crisis in North Korea. Beijing has no interest in planning with Washington and Seoul, thinking such talks too sensitive. And Seoul worries that such talks would cause tensions with Beijing to spike.

Beyond that, the US and China have pursued far different priorities with the North in recent years. Washington wants denuclearisation. Beijing wants influence. Scott Snyder, a senior fellow for Korea studies at the Council on Foreign Relations, said the report provides "an interesting effort to correct Washington's tendency to view the [North Korea] issue primarily with a denuclearisation focus without considering geostrategy".

China now accounts for 70% of North Korea's trade, filling a void that opened several years ago when Seoul, under a new conservative government, drew a harder line with the North and drastically cut back on its economic co-operation. Scholars now commonly describe North Korea as a Chinese vassal state – though they note that the North, famous for its propaganda emphasising self-reliance, bristles at its dependence on Beijing.

In recent years, China has pumped billions in investment into the North, building up its roads and ports, helping the North in several special economic zones, and importing North Korea's considerable reserve of natural resources and rare earths.

Luse's report shows the danger of this: China has made a "tributary province" of the North, while South Korea has ceded ground. Still, others point out that China's growing investment in the North also presents a potential upside for the region, if ever the North loosens tight state control over its economy and allows greater freedom for its people.

Chinese government officials issue frequent calls for stability on the Korean peninsula, and for the sake of maintaining that stability, they've tried to block additional and tighter international sanctions during the past three years after North Korean military strikes and long-range rocket launches. That protection, coupled with the economic investment, acts essentially as a Chinese-led bailout for the North Korean government, allowing it to survive after earlier sanctions were imposed in 2006 and 2009.

Some Chinese experts note that a unified Korean peninsula – democratic, under a government in Seoul – represents an even greater form of stability.

"A peaceful Korean peninsula is in line with China's national interest," said Zhang Liangui, a professor of international strategic research at the Party School of the China Communist Party Central Committee, who noted that a unified Korea would allow for even deeper Chinese investment.

But he added that within China, opinion on how to handle the Korean peninsula is sharply divided. "One misunderstanding of the US is that they think of China as a whole," its leaders in agreement on everything.

The Senate report devotes an extensive section to China's relatively recent assertion of old territorial claims and says that China "may be seeking to lay the groundwork for possible future territorial claims on the Korean peninsula".

One state-sponsored research project mentioned in the report, published in 2003, makes the controversial case that an ancient kingdom operating more than 1,300 years ago on the Korean peninsula was under Chinese control. Another major Chinese atlas says that Chinese territory once descended across the western side of the Korean peninsula, toward the southern tip.

This article appeared in the Guardian Weekly, which incorporates material from the Washington Post