Rwanda has deployed what it called a “protection force” to the Central African Republic after its peacekeepers there were attacked by rebels advancing to the capital, Bangui.
The Rwandan and CAR governments have accused ex-President François Bozizé of backing the rebels and plotting a coup, which he has denied.
UN peacekeepers have said the rebel advance has been halted.
Mr Bozizé has been barred from a presidential election due on Sunday.
Russia has also sent “several hundred soldiers and heavy weapons” to the country to support the government, the AFP news agency quotes a CAR government spokesman as saying.
The spokesman, Ange Maxime Kazagui, said the Russian forces were invited as part of bilateral agreements, AFP reports.
The Russian government has not confirmed these reports.
“The Rwandans have also sent several hundred men who are on the ground and have started fighting,” Mr Kazagui added.
The number of additional Rwandan soldiers deployed has not been given but the defence ministry said the operation was done “under existing bilateral agreements” with CAR.
At least 750 Rwandan soldiers and police officers have been operating under the UN peace-keeping force Minusca.
Minusca forces have also been deployed beyond Bangui “to block armed elements”, AFP quotes a UN spokesman as saying.
President Faustin Archange Touadéra has insisted Sunday’s election will go ahead, saying the presence of the army and UN peacekeepers means people have nothing to fear.
But opposition parties, including that of Mr Bozizé, have called for the vote to be postponed “until the re-establishment of peace and security”.
Rebel groups have seized several towns close to CAR’s capital, clashing with government forces and looting property, and the UN said its troops were working to prevent a blockade of Bangui.
Mr Bozizé’s spokesman Christian Guenebem said: “We categorically deny that Bozizé is at the origin of anything.”