Fighting between government and rebel forces in eastern Democratic Republic of Congo has continued for a second day, near the regional capital, Goma.
Four government soldiers and 15 rebels died on Monday, government spokesperson Lambert Mende told the BBC.
These are the first clashes between the two sides since the rebels pulled out of Goma last year.
The UN said it would speed up the deployment of a new intervention force in response to the violence.
A 3,000-strong brigade is being sent to neutralise and disarm the rebels, with the first contingent arriving last week.
"Considering what has happened I think we must expedite the deployment so they will be fully responsible as soon as possible," UN Secretary-General Ban Ki-moon told AFP.
The BBC's Maud Jullien in the capital, Kinshasa, says the government believes the M23 is trying to disrupt the deployment of the UN force.
Mr Ban is due to visit DR Congo, Rwanda and Uganda later this week.
The two sides exchanged artillery and mortar fire north of the city of Goma, with each accusing the other of provoking the clashes.
Fighting in Mutaho, 10km (6 miles) east of Goma, broke out early on Tuesday after M23 rebels tried to take back positions lost to the army on Monday, Col Olivier Hamuli told AP.
In a statement to AFP, M23 denounced the "military option" taken by the government and reaffirmed its commitment to peace talks.
Peace talks began following the withdrawal of M23 troops from Goma in December but have stalled in recent months.
On Monday, Rwandan President Paul Kagame told the BBC that UN troops had "in some cases" made the situation in DR Congo worse.
He said any military effort to bring peace to DR Congo needed to be "properly co-ordinated" with political efforts.
Mr Kagame dismissed long-standing UN allegations that Rwanda backed the M23.
Some 800,000 people have fled fighting since the M23 launched its rebellion last year.