In a telephone call on Sunday, Blinken "made clear that any external support to non-state armed groups in the DRC must end, including Rwanda's assistance to M23, an armed group that has been designated by the United States and the United Nations," spokesman Ned Price said.
Blinken "shared deep concern about the impact of the fighting on Congolese civilians who have been killed, injured and displaced from their homes," Price said in a statement.
The top US diplomat urged both countries to adhere to ceasefire agreement negotiated last month by Angola.
Blinken also condemned "the resurgence of hate speech and public incitement" targeting Rwanda, recalling the "real and horrible consequences of such rhetoric in the past," an allusion to the 1994 genocide that mostly targeted Tutsis.
Rwanda has denied repeated US-backed accounts of support for the M23, a mostly DRC Tutsi group that resumed fighting in late 2021 after lying dormant for years.
After a brief period of calm, it went on the offensive in October and has advanced toward the eastern regional hub of Goma.
Kinshasa last week accused the M23 of slaughtering more than 100 people in the village of Kinishe, around 70 kilometers north of Goma, charges denied by the militia.
Blinken aired criticism about Rwanda's role in DR Congo during visits in August to the neighboring countries.