NAIROBI, Kenya — Hundreds of troops from opposing sides have been moving into new positions around Goma, a strategic, contested city in the Democratic Republic of Congo, aid organizations said Sunday, raising worries of yet another explosion of violence.

Witnesses reported that truckloads of Rwandan soldiers recently crossed the border and were camped within 20 miles of Goma, which was captured last month and briefly occupied by a rebel force called the M23 movement. United Nations officials confirmed a sudden military buildup around Goma but said that they did not know the identities of the various groups of soldiers.

“It feels a bit like a boiling kettle, about to blow up,” said Thierry Goffeau, the leader of the mission in Goma for the aid organization Doctors Without Borders.

The M23 rebels are widely believed to be covertly armed by neighboring Rwanda, and in the past few months they have eviscerated the Congolese Army in just about every battle.