Johnnie Carson, the assistant secretary of state for African affairs, said the United States has worked closely with other countries to try to end the hostilities, with officials from Secretary of State Hillary Rodham Clinton to U.N. Ambassador Susan Rice pursuing a comprehensive response.
Carson described the situation in Congo as the most volatile in Africa, with an estimated 5 million people killed since a second regional war began in 1997. Despite a major peacekeeping operation, the M23 rebel group is operating in eastern Congo.
Rep. Chris Smith, R-N.J., chairman of the House Foreign Affairs subcommittee that oversees Africa, called it an "ongoing reign of terror," and questioned whether administration's policy had failed.
"I reject that notion and reject that pretty soundly," Carson told the subcommittee. "At all levels of the U.S. government, we are working to advance greater peace and stability."
Carson detailed various trips and outreach by Clinton, Rice and Undersecretary Wendy Sherman. A recent U.N. report accused the Rwandan government of backing the rebels, a charge Rwanda denies. The insurgents have been accused of rape, using child soldiers and conducting summary executions in eastern Congo.
The situation has created greater scrutiny of Rice, a potential candidate to succeed Clinton, and her relationship with Rwandan President Paul Kagame. Rice served as assistant secretary for African affairs and on the National Security Council in the Clinton administration during the effort to end the genocide in Rwanda in the 1990s.
Carson said that in the past six months, the United States had reduced its foreign military assistance to Rwanda by $200,000, although it is still maintaining developmental aid.
Lawmakers on the subcommittee scoffed at that cut.
"That's a drop in the bucket," said Rep. Tom Marino, R-Pa., who said the United States has provided some $1 billion in aid to Rwanda in the past decade. "It doesn't seem that we're very serious about this."
Carson said the United States had no evidence that Rwanda was misusing the money or that it was ending up in the hands of the rebels.
"Rwanda has a high level of credibility with respect to the way it uses its resources," he said.
The testimony failed to satisfy the lawmakers.
"How many people have to die before you stop and get serious about this?" Marino said.
Separately, 13 House members led by Democrats Howard Berman, Karen Bass and Adam Smith sent a letter to President Barack Obama pleading for the appointment of a presidential envoy, backing for a U.N. envoy and calling for the African Union to appoint a special emissary.
"M23's formation and inexcusable military advance, supported by Rwanda and to a lesser extent Uganda, is only the latest chapter in Central Africa's conflict," the lawmakers wrote. "We feel the systemic problems that drive the cyclical fighting can be broken, but only if the political leaders of Central African country governments take decisive and sustained action and the international community maintains a heavy focus on these issues with sustained high-level leadership."
Human rights groups such as the Enough Project, Freedom House and Invisible Children sent a letter to Obama on Monday also pressing for an envoy and a cutoff of aid to Rwanda.