Rwandan President Paul Kagame said on Wednesday he was ready to send troops back into the Democratic Republic of Congo if it fails to rein in militias.
But he said that for now he was waiting for the results of landmark elections in the war-torn neighbouring country.
"If we are attacked by militias or by anybody, we will have the right" and "capacity to deal with the problem," Kagame said on a visit to Japan.
"We will do whatever any country would do if it was attacked. It is as simple as that," he told reporters at the United Nations University in Tokyo.
But he said he was carefully watching results from the Democratic Republic of Congo's (DRC) first free elections in four decades. Results from the run-off election are expected on November 19.
"It is possible to find another way of dealing with the problem," even if militia attacks occur, "especially working with the government of Congo, in the DRC, as they are going through elections," he said.
Rwanda, along with Burundi, the DRC and Uganda, are all finding ways to address rebel groups in the lawless eastern DRC that have refused to disarm themselves.
Groups include the Democratic Forces for the Liberation of Rwanda, which includes Hutu former soldiers accused of participating in the country's 1994 genocide and vehemently opposed to Kagame's Tutsi-led government.
The DRC government's army for years openly supported the Hutu militia who provided crucial support during the country's 1998-2003 war. The recent elections are meant to complete the transition from the conflict to peace.
Kagame renewed his criticism of the International Criminal Tribunal of Rwanda, saying it was doing too little to try genocide leaders, although he thanked international support for it.
"The ICTR has for example spent a billion dollars, US dollars, only managing to try about 32 cases," Kagame said. It "sounds ridiculous to me."
Around 800,000 mainly Tutsi people -- about a tenth of the country's population -- were killed within six weeks in Rwanda in 1994 by Hutu extremists.
The Rwandan president is visiting Japan on his way back from a conference in China and met later Wednesday with Prime Minister Shinzo Abe.
Kagame's visit to Japan is the first by a Rwandan president in 10 years.
Japan has stepped up aid to Africa in recent years as it tries to secure support from the continent for its cherished but increasing distant goal of a permanent seat on the UN Security Council.