Reporters saw the Deputy Interior Minister, Faustin Munene, travelling in a column of half-a-dozen tanks and 20 trucks filled with troops heading towards Mbanza Ngungu, 120km from Kinshasa.
Rebel commanders said their advance units were approaching the former slave-trading garrison town, where the former Zaire's armoured units were traditionally based.
At the weekend witnesses said government forces were withdrawing "in haste" from Songololo, south-west of Mbanza Ngungu, but fresh troops and armour were seen heading back towards there on Monday. It was not clear who controlled Songololo.
In Mbanza Ngungu, volunteers, responding to an earlier call from President Laurent Kabila to sign up to defend their homeland, were manning roadblocks and other reinforcements were in place. Mr Kabila's government conceded on Monday that the Rwandan- backed rebels held the Inga dam, in the western war zone.
"We have what is needed to stop them reaching Kinshasa," the Information Minister, Didier Mumengi, told a news conference in Kinshasa on Monday. "I can assure you that they will never reach Kinshasa."
On Sunday, Reuters correspondents reported from the Congo River port of Matadi, near the border with Angola, and Inga that both were under rebel control. Government soldiers retreating from Songololo at the weekend said a small town, Kenge, between there and Matadi, had fallen to rebels.
The proximity of the fighting to Angola, where relations between the government of President Jose Eduardo dos Santos and former Unita rebels have soured despite a peace deal, has fuelled speculation of Angolan involvement. Retreating soldiers said Unita fighters were advancing alongside the rebels. There was no independent confirmation. Mr Kabila accuses his former allies Rwanda and Uganda, which helped him oust the dictator Mobutu Sese Seko in May 1997, of fighting alongside ethnic Tutsi and other rebels who took up arms in the east on 2 August.
He fell out with his former sponsors over ways of ensuring security and stability in the Congo and Central Africa as a whole, and over the status of Tutsis in Congo. Both countries deny it but independent sources say Rwandan forces are involved in the latest revolt.
Mr Kabila held weekend talks in Angola's capital, Luanda, with Mr Dos Santos and Namibia's President, Sam Nujoma.
He left for the southern city of Lubumbashi on Monday amid speculation that he was seeking help from Angola and other friends.
Angola's government, whose Unita rivals operated from the former Zaire under Mobutu, played a pivotal role in a civil war in Brazzaville, the Congo Republic capital across the Congo River from Kinshasa, when its intervention brought the former military ruler, Denis Sassou Nguesso, back to power in October.
Mr Mumengi declined to say whether Mr Kabila went to Harare to join a meeting on Monday of defence ministers from Angola, Zimbabwe and Namibia. Diplomats in Harare said Mr Kabila had sought air support from Angola, troops from Zimbabwe and logistical support from Namibia. Western diplomats said Angola and the others might want to assess Mr Kabila's survival chances before committing themselves to a conflict which would sow division across much of central, southern and eastern Africa.
"For Luanda, the Congolese conflict is only important in so far as it impacts on their ability to combat Unita," a diplomat said, referring to the former rebel movement and fragmenting peace process between the government and Unita.
Traditionally, the 1,500km border that Angola and Congo share has been a key entry point for supplies to Unita.
In sending troops last year to help Mr Kabila oust Mobutu, who was one of Unita's main allies, the Angolan government hoped to sever the movement's supply lines. "The Angolans invested a lot in putting Kabila in power, but he has proved ineffectual against Unita," said the diplomat.
"He has not moved against them, and he has not allowed the Angolan government to send troops through Congo in a pincer movement, as they had hoped."
The diplomats said Angola was likely to maintain relations with both sides in the conflict at least until it was clear whether a better alternative to Mr Kabila was on the horizon.