Speaking on condition of anonymity, a UN source and an aid worker told AFP that large numbers of soldiers entered DR Congo at the Nobili border post in North Kivu state, part of the country's east plagued by conflict involving the ADF armed group.

"It's a column of very well-armed troops on foot, followed by armoured vehicles," the aid worker said.

Video shared on social media also showed advancing soldiers whose uniforms bore the Ugandan flag.

Eyewitnesses had earlier reported explosions and artillery fire in North Kivu's Watalinga district, as well as the Boga and Tchabi districts -- known hideouts of the ADF in neighbouring Ituri province.

Both Uganda and the Democratic Republic of Congo had announced joint artillery and air strikes, but had yet to confirm the arrival of Ugandan ground troops by the end of the day.

"As announced, targeted and concerted action with the Ugandan army started today with air strikes and artillery fire from Uganda against positions of the terrorist ADF in the DRC," Congolese government spokesman and Communications Minister Patrick Muyaya wrote on Twitter.

Ugandan army spokeswoman Flavia Byekwaso said in a statement that "targets were accurately hit" earlier Tuesday.

She later told AFP that "after the air strikes ground operations will follow. There are going to be ground troops moving in."

So far Congolese authorities have not confirmed any Ugandan movement into the country, and Muyaya had said Monday there were no ground forces present.

But the strikes came two days after a senior Congolese source reported that President Felix Tshisekedi had given Uganda permission to pursue the ADF on DR Congo soil.

- Thousands killed -

The ADF, linked by the United States to the Islamic State group, is deeply feared in eastern Democratic Republic of Congo.

The DR Congo's Catholic Church says the ADF has killed around 6,000 civilians since 2013 while a respected monitor, the Kivu Security Tracker, blames it for more than 1,200 deaths in North Kivu's Beni area alone since 2017.

The Ugandan authorities recently accused the ADF or a local group affiliated with it of carrying out or planning attacks.

The ADF was historically a Ugandan rebel coalition whose biggest group comprised Muslims opposed to Ugandan President Yoweri Museveni.

The group established itself in eastern DRC in 1995, later becoming the deadliest of scores of outlawed forces in the troubled region.

Since April 2019, some ADF attacks have been claimed by IS, which describes the group as its Islamic State Central Africa Province offshoot.

In March, the United States placed the ADF on its list of "terrorist" organisations linked to the IS.

- Ugandan attacks -

On November 16, four people were killed and 33 were injured in twin suicide bombings in the Ugandan capital Kampala, which police attributed to a "domestic terror group" linked to the ADF.

The blasts, claimed by IS, came on the heels of a bomb attack at a roadside eatery on October 23 that killed one woman, and a suicide blast on a bus near Kampala on October 25 that wounded several people.

In late October, the Ugandan police said they had arrested suspected ADF members, who they believed were plotting a new attack on "major installations".

Uganda has also blamed the ADF for a foiled bomb attack in August on the funeral of an army commander who led a major offensive against Al-Shabaab militants in Somalia.

Three men were charged with terrorism on November 5 in relation to that incident.

A Congolese presidential advisor in Kinshasa told AFP on Sunday that the DRC would allow armed forces from Uganda to "enter Congolese territory to chase ADF terrorists."

The move is not universally supported in the DRC, where many critics recall the role of Uganda and Rwanda in the decades-long instability in the east of the country.

The eastern DRC provinces of North Kivu and Ituri have been under a "state of siege" since May in a bid to step up a military offensive against the rebels, with soldiers replacing civil servants in key positions.