The UN special force in eastern Democratic Republic of Congo engaged in direct combat with besieged M23 rebels, throwing its weight behind a crushing army assault despite calls for a truce.
A UN source said the intervention brigade fired mortar rounds after getting the "green light" to bombard the last positions of the M23, in what appeared to be the first time the force has actively taken part in the fighting.
"Our targets are... Runyonyi and Chanzu," said another source in the UN brigade, referring to two hilltops about 80 kilometres north of regional capital Goma, at about 2,000 metres (6,500 feet), where dozens of holdout rebels have dug in.
"There is some resistance. We will continue to fire until everything is under control," added the source.
Until now, the UN intervention brigade had been backing the Congolese forces with aerial reconnaissance, intelligence and planning.
"It is important that the M23... declare the end of the rebellion. The fighting must stop," Said Martin Kobler, head of the UN mission in DR Congo (MONUSCO), in a statement.
News of the UN force's intervention came hours after the Congolese army said it had seized a key rebel position, the latest in a string of military victories over the rebels since a major offensive was launched on October 25.
A senior official said Congolese troops had "completely conquered" the Mbuzi hilltop, as fighting raged in the mountainous region where rebels have retreated to since being forced from their final stronghold of Bunagana last week.
The latest clashes came despite a ceasefire call by M23 leader Bertrand Bisimwa on Sunday, which prompted international calls for Kinshasa to stop pressing on with the offensive.
Urging rebels to "immediately end hostilities", Bisimwa said his aim was to "allow the continuation of the political process" with Kinshasa in a bid to end the insurgency plaguing the long-troubled region since April 2012.
But the fighting only appeared to intensify after the M23 leader's appeal, with AFP journalists reporting that heavy shelling could be heard Monday in the small town of Ntamugenga near the battle zone in the lush, hilly region bordering Uganda.
"Victory, Victory," soldiers cried at a post in the town after receiving a radio message that their colleagues had taken Mbuzi.
Thousands flee into Uganda
The governor of the DR Congo's eastern North Kivu province, Julien Paluku, said shelling had left six people dead in Bunagana, a town on the border with Uganda.
Ugandan army colonel Paddy Ankunda said some shells had landed on its territory but there were no injuries or plans for any military response from Kampala.
The UN refugee agency meanwhile said it had moved another 3,000 Congolese refugees to its transit camp in Kisoro to escape the fighting, bringing the total number of refugees in the small Ugandan town to 8,000.
African leaders met in Pretoria in South Africa and late on Monday said a peace deal for DR Congo could be signed if the M23 declare an end to their rebellion.
"The joint summit noted that all the 11 issues under discussion in the Kampala dialogue had been agreed upon and that the parties would sign an agreement on condition that the M23 makes a public declaration renouncing rebellion," said Stergomena Tax, executive secretary of the 15-country Southern African Development Community (SADC) in Pretoria.
The joint summit between SADC and some of DR Congo's neighbours also commended the Congolese army and the intervention brigade "for recapturing M23 strongholds and restoring government control," Tax said.
The heavily-armed 3,000-strong UN intervention brigade joined 17,000 peacekeepers already deployed with a mission to carry out offensive operations against the rebel fighters, who are accused of human rights abuses including rape, murder and recruiting child soldiers.
In a joint statement issued early Monday, envoys from the European Union, African Union and the United Nations said they were "concerned about the renewed outbreak of violence" that followed the M23 truce call.
Analysts say better preparation by the Congolese troops and the backing of the UN brigade with the unprecedented offensive mandate have helped changed the game in the restive east of the DR Congo.
The M23 movement was founded by ethnic Tutsi former rebels who were incorporated into the Congolese army under a 2009 peace deal but mutinied in April 2012, claiming the pact had never been fully implemented.