UN peacekeeping troops are backing a Congolese army drive against jungle-based rebel groups that is expected to displace at least 100,000 people and trigger a new wave of instability and human rights abuses across war-ravaged eastern Congo, aid workers and independent analysts have warned.

The new offensive by 5,400 troops of the Democratic Republic of Congo army (FARDC), largely unreported until now, began in South Kivu province, bordering Rwanda and Burundi, on 15 February and is being extended into North Kivu, bordering Uganda, this month.

The push into remote areas in the west and north of the two provinces is targeting the Rwandan Hutu rebel group the FDLR (Democratic Forces for the Liberation of Rwanda), local armed groups known as mai-mai, and remnants of Joseph Kony's Lord's Resistance Army, sources said.

Fears are growing that civilians will be caught up in the offensive, dubbed Operation Perfect Peace, and that the fragile calm of the past year, when the FARDC withdrew many of its regiments for reorganisation and retraining, will be shattered.

"The international community is struggling to keep a lid on eastern Congo," said Anais Lafite, Oxfam's provincial co-ordinator for South Kivu, based in Bukavu. "They are trying to maintain the status quo for fear that worse might follow … About 100,000 people have already been displaced since last October. It's estimated the current operation could displace a further 100,000."

The last joint UN-Congolese army offensive aimed at the FDLR, in 2009, was blamed for contributing to more than 1,000 civilian deaths, the mass rape of more than 4,500 women, and the forced displacement of more than 800,000 people. Despite a western-assisted reform programme, the Congolese army has a reputation as one of the world's worst human rights abusers.

FDLR fighters have killed at least 45 civilians since January, and thousands of people have fled across the border into Uganda amid reports of rebel reprisals in response to the army attack. Sources said 12,000 people were displaced last week by escalating military clashes in North Kivu, home to an estimated 600,000 internally displaced people, of whom there are 1.7 million in eastern Congo, plus hundreds of thousands in neighbouring countries.

The renewed fighting comes amid acute political uncertainty in Kinshasa following last November's disputed presidential and parliamentary elections, won by President Joseph Kabila and his allies.

Apparently smarting over allegations of vote rigging made by the US and the Catholic church, which accused his government of "treachery, lies and terror", Kabila has not been seen in public for two months. A new government has yet to be formed.

"The UN is being sucked in and the new offensive risks making things worse," said Alain Salesse, an independent consultant based in Bukavu. "The security and political situation is deteriorating. The mai-mai are waking up and there is more Rwandan interference."

Backing for the operation from the UN force in Congo, known as Monusco, includes provision of troop transport and helicopter support, planning and tactical assistance, fuel, food and other logistical help. Monusco is the UN's largest peacekeeping/stabilisation mission, with 17,000 troops on the ground, 20,000 personnel in total, and an annual budget of $1.5bn (£947m). It recently took delivery of new attack helicopters from Ukraine.

The UN troops are not expected to take part directly in the fighting, but Monusco's security council mandate is termed "robust" and allows it to use "all necessary means" to ensure the protection of civilians, its top priority.

A senior UN official in Goma, capital of North Kivu, where Monusco is based, said Pakistani UN troops became involved in fighting during the 2009 offensive and the reality was that they would again. The UN mission is also tasked with strengthening state authority in the DRC. But this role, too, is increasingly controversial given strong international criticism of irregularities in November's elections. Kabila's inauguration was boycotted by foreign leaders. Zimbabwe's president, Robert Mugabe, was the only head of state to attend.

In its annual report, Human Rights Watch described the human rights situation in the DRC as "grave". It said: "The east and the north of the country remained volatile and were marked by frequent attacks on civilians, particularly sexual violence against women and girls … As in the past, government soldiers frequently killed and raped civilians and pressed them into forced labour or looted their belongings."

A local aid worker with long experience of the country said: "Government corruption is totally endemic. The kickback culture is everywhere." Government ministers' main interest in eastern Congo was exploiting its gold and other mineral riches, often in concert with Rwandophone groups such as the National Congress for the Defence of the People (CNDP) armed militia, the aid worker said.

Laurent Nkunda, former leader of the CNDP, who is associated with a series of atrocities, is believed to be under house arrest in Rwanda. The DRC wants him extradited to face war crimes charges but so far Kigali has refused.

Meanwhile, an allegedly Rwandan-backed splinter faction led by Bosco Ntaganda continues to operate out of Goma, running illegal mining rackets undisturbed by the Kabila government in Kinshasa.

Ntaganda is wanted by the international criminal court on charges of conscripting child soldiers. But government officials have claimed they fear his arrest could disrupt the integration of former rebel groups, including the CNDP, into the Congolese army, and harm relations with Rwanda.

Monusco says it is screening the Congolese army soldiers it will work with to try to ensure joint operations do not include any former human rights abusers. Human rights monitors and other civilians will also form part of the operation. But rights workers say it remains unclear whether these precautions will make any difference. Renewed military action against armed groups in eastern DRC comes despite repeated warnings from civil society organisations that the operations are unlikely to address the root causes of the region's twin humanitarian and developmental crises or resolve the widespread proliferation of armed groups.

Monusco's mandate prioritises the protection of civilians. But it is also tasked with strengthening state authority. The inherent tension between these two aims is now being played out in the Kivus.

Monusco headquarters in Kinshasa did not respond to repeated requests for comment about the joint operations in the Kivus.