Rwanda is reported to have armed and trained refugees to fight on behalf of the Burundian opposition.
The Rwandan government has denied the allegations.
Burundi has been hit by civil conflict since President Pierre Nkurunziza decided to stand for a third term.
In a hearing in the US Senate Foreign Relations Committee, two top diplomats cited reports from colleagues in the field that they said pointed to Rwandan involvement in the Burundi crisis.
Thomas Perriello, US envoy for the Great Lakes region of Africa, said the reports suggested that Burundian refugees, including children, were being recruited from camps in Rwanda to participate in armed attacks against the Burundian government.
Turmoil erupted in Burundi after Mr Nkurunziza announced plans last April to run for a third term, which he went on to win.
More than 400 people have died in the violence and at least 240,000 have fled the country.
Linda Thomas-Greenfield, US assistant secretary of African affairs, said that US officials had encouraged Rwandan authorities “to play a productive role and not to do anything that might further destabilise Burundi”.
The governments of Rwanda and Burundi are from rival ethnic groups and there is growing international concern that another ethnic conflict could take root in the region.
Last week, a UN panel reported that Burundian refugees had been recruited at a refugee camp in eastern Rwanda in May and June 2015, and given two months of military training.
Linda Thomas-Greenfield, US assistant secretary of African affairs, said that US officials had encouraged Rwandan authorities “to play a productive role and not to do anything that might further destabilise Burundi”.
The governments of Rwanda and Burundi are from rival ethnic groups and there is growing international concern that another ethnic conflict could take root in the region.
Last week, a UN panel reported that Burundian refugees had been recruited at a refugee camp in eastern Rwanda in May and June 2015, and given two months of military training.