Frustrated that China had not done more to rein in North Korea, the Trump administration considered new sanctions in July on small Chinese banks and other firms doing business with Pyongyang, two senior United States officials told Reuters.
Two earthquakes were detected near North Korea's nuclear test site in Punggye-ri, North Hamgyong Province on Saturday afternoon.
North Korea's foreign minister, Ri Yong Ho, said Thursday that the country could soon test a hydrogen bomb over the Pacific Ocean.
The Comprehensive Nuclear-Test-Ban Treaty Organization, through its executive secretary, tweeted Saturday that the activity was "unlikely man-made".
The majority of North Korea's oil likely comes from China, but the exact tally of oil exports remains unknown, as Beijing has not published such data since 2014.
The US has accused Beijing of not doing enough to pressure Pyongyang into abandoning its nuclear programme. The agency said that while it was analyzing the nature of the quake, its initial view was that it was a natural earthquake.
Trump and North Korean leader Kim Jong Un exchanged personal insults this week, and neither side shows any sign of backing down.
Until now, the North has not tested its missiles and nuclear weapons together, opting instead to fire them separately - the missiles above ground and the nuclear weapons below.
The magnitude 2.6 and 3.2 tremors occurred 6 km north-northwest of the nuclear test site at 1:43 p.m. and 5:29 p.m. Saturday, according to the Korea Meteorological Administration.
Trump has dubbed Kim a "madman" and sought to ratchet up sanctions against the isolated regime, which says it needs nuclear weapons to protect itself against the threat of invasion.
The U.S. Geological Survey said that it detected a magnitude 3.5 quake in the area of previous North Korean nuclear tests, but that it was unable to confirm whether the event was natural.
Kim responded to Trump's remarks at the UN General Assembly in a statement on state media, calling the president "mentally deranged" and threatening to take the "highest level of hard-line countermeasure in history".
The sources said banks were told to stop providing financial services to new North Korean customers and to wind down loans with existing customers, following tighter sanctions against Pyongyang by the United Nations.