Uganda has deployed more than 1 000 extra soldiers into east Congo in the last week near an area where the Kinshasa government is fighting M23 rebels, four diplomatic and United Nations (UN) sources said, heightening fears of a regional escalation.

That raised the number of Ugandan soldiers, officially there to back Democratic Republic of Congo President Felix Tshisekedi's government, to about 4 000-5 000, according to UN sources. Rwanda backs the M23 and also has troops in east Congo.

Uganda has been helping Congo's army against another rebel force, the Islamist Allied Defence Forces, and the new deployment of between 1 000-2 000 troops was under that auspice in a push called Operation Shujaa, the sources said.

In a region of complex and often-shifting alliances, UN experts say Uganda has in the past also backed the M23, which is the latest in a string of Rwanda-backed rebellions to take up arms in the name of Congo's ethnic Tutsis.

Ugandan army spokesperson Felix Kulayigye denied a major new deployment, saying its forces had changed their "posture to offensive defence", without giving further details.

Congo's Communications Minister Patrick Muyaya did not respond when asked if more troops had arrived, but stressed the priority of Ugandan soldiers in the area was to fight the ADF though combat with M23 and Rwandan soldiers was also possible.

"There's still a lot of suspicion about Uganda, a lot of suspicion about what's generally happening with the M23," he added.