Explainer: Why is there more fighting in eastern Congo?

(Reuters) - Democratic Republic of Congo's army has been engaged in heavy fighting since the weekend against the M23 rebel group, which is waging its most sustained offensive since a 2012-2013 insurrection that captured vast swathes of territory.

The rebels control positions as close as 20 km (12 miles) to eastern Congo's main city of Goma and briefly took over the main military base in the province, local officials said.

The fighting has forced tens of thousands to flee their homes in a region that has had little respite from conflict ever since neighbours Rwanda and Uganda invaded in 1996, citing threats from local militia groups.

WHO ARE THE M23?

When it formed in 2012, M23 was the newest in a series of ethnic Tutsi-led insurgencies to rise up against Congolese forces.

M23's name refers to the March 23 date of a 2009 accord that ended a previous Tutsi-led revolt in eastern Congo. The M23 accused the authorities of not living up to promises to fully integrate Congolese Tutsis into the army and government.

The M23 and its predecessor groups have claimed to defend Tutsi interests, particularly against ethnic Hutu militias like the Democratic Forces for the Liberation of Rwanda (FDLR).

The FDLR was founded by Hutus who fled Rwanda after participating in the 1994 genocide of more than 800,000 Tutsis and moderate Hutus.

WHAT HAPPENED IN 2012-2013?

M23 fighters seized vast swathes of the eastern Congo countryside in 2012, routing under-equipped army soldiers. In November, the group briefly captured Goma, a city of 1 million people.

That led to an overhaul of the Congolese army and the deployment of a special Force Intervention Brigade of United Nations peacekeepers, changing the tide of the war.

By late 2013, the Congolese and U.N. forces had chased the remnants of the group into Rwanda and Uganda.

That December, Congo's government and the M23 signed a peace accord. The M23 agreed to transform itself into a political party and Congo promised amnesty for most fighters as part of a disarmament and demobilisation process.

WHY IS THE M23 FIGHTING AGAIN?

Discontent has been building among the M23's ranks. The group has complained that Congolese authorities have been slow to grant amnesties and propose economic opportunities in Congo to its fighters stuck in Uganda and Rwanda.

Groups of M23 fighters have staged small-scale attacks inside Congo over the past several years.

The M23 says its current military activities are defensive in nature following attacks by the FDLR, which it says is collaborating with the Congolese army. The army has denied working with the FDLR.

M23 spokesman Willy Ngoma said on Thursday that the group was satisfied with its current territorial position but that it could try to seize additional territory, including Goma, if that were "necessary for our defence".

ARE CONGO'S NEIGHBOURS INVOLVED?

Congo's eastern neighbours, particularly Rwanda and Uganda, have a long history of military intervention inside Congo. The two countries invaded in 1996, and again in 1998, claiming they were defending themselves against local militia groups.

Though the latter of those wars ended with a peace treaty in 2003, Congo's government, U.N. investigators and independent experts have, in the years since, accused Rwanda and Uganda of supporting militias inside Congo, including the M23.

They say the support has been aimed at maintaining geopolitical influence and profiting from extraction of the area's mineral riches.

Both countries have repeatedly denied those charges.

Rwandan President Paul Kagame suggested in February that Rwandan forces might need to intervene in eastern Congo because of the threat from Hutu militiamen.

On Wednesday, Congo accused Rwanda of backing the M23's latest offensive, citing the rebels' heavy firepower as evidence of outside support. Rwanda denied this, calling the fighting an intra-Congolese conflict.

(Reporting by Aaron Ross; Editing by Catherine Evans)