A U.S. State Department official told a Senate subcommittee this week the government may probe deeper into nuclear-capable North Korea’s chief source of foreign help. Aid is apparently coming from a country that’s normally careful neither to condemn nor condone North Koreannuclear tests that scare U.S. allies such as Japan and South Korea. Last month the angry hermit government in Pyongyang tested a submarine-launched ballistic missile that riled Japan and the United States. UNsanctions against North Korea and backed Washington forbid trade in military supplies.

The State Department may look harder at Chinese companies in case they are equipping North Korea’s nuclear development, sanctions coordinator Daniel Fried told the foreign affairs subcommittee Wednesday. His comment followed an announcement two days earlier that four Chinese nationals and Chinese industrial equipment wholesaler Dandong Hongxiang Industrial Co. faced their own sanctions and money laundering charges over suspected military support for North Korea via U.S. financial institutions.

But a broader probe would just turn up just one thing for sure: China is North Korea’s BFF, a steadfast ally that sanctions probably can’t break.

More vendors, perhaps concentrated along the Chinese-North Korean border, might fall into the U.S. government net. But they would number too few to ripple into China’s broader economy or prompt much reaction from Beijing, analysts say.

The lack of reaction would point to China’s quiet but unwavering support for North Korea. China hopes to keep the fellow Communist nation in its pocket as a geographic buffer against democratic, Western-backed countries such as South Korea. Economic collapse in already impoverished North Korea could suddenly change the Beijing-sympathetic regime and may send refugees over the border to China.

“China’s concerns are entirely strategic. They worry that squeezing North Korea too hard would precipitate a crisis, either a military confrontation or North Korea’s economic collapse,” says Scott Kennedy, director with the Project on Chinese Business & Political Economy under American think tank Center for Strategic & International Studies. “Avoiding either outcome is their top priority.”