The Daily Telegraph said the first signs of sexual exploitation of local youths in southern Sudan emerged within months of the peacekeepers' arrival in March 2005.
The UN Children's Fund, Unicef, drafted an internal report detailing the problem, it said.
But Montas said the Unicef report dealt only with abuses by the Sudanese military, not UN peacekeepers.
She said that the UN had been aware of the charges of abuse for some time and was working with local authorities and countries contributing troops to the UN mission in southern Sudan "to ensure that UN personnel adhere to the highest standards of accountability".
"When necessary, strong disciplinary action will be taken.
"The UN standard on this issue is clear - zero tolerance, meaning zero complacency and zero impunity."
The newspaper report appeared on the first day of work for Ban Ki-moon, the new UN secretary-general, of South Korea.
There are more than 11,000 UN peacekeepers and police from some 70 countries in southern Sudan, enforcing a January 2005 peace agreement that ended a 21-year civil war.
|Ban faced a busy first day at work [AFP]|
The UN, working with the African Union, is now pressing a reluctant Sudanese government to admit thousands more peacekeepers to its western Darfur region, where a separate conflict has raged for three years.
Sexual abuse charges have surfaced for decades in UN peacekeeping missions and among civilian and other humanitarian staff operating around the world.
Since January 2004, the UN has investigated abuse allegations against 319 military and civilian personnel in all its missions, the world body said in late November.
It has disciplined 179 soldiers, civilians and police since then but acknowledges minors and the poor are still exploited despite its best efforts.