This was the first major ceasefire violation in a year
In a unanimous vote, the Security Council condemned the Rwandan-backed RCD-Goma faction for its action, saying this was a major violation of the ceasefire.
The RCD, for its part, says it welcomes the resolution and intends to hand over control of Moliro to the UN mission in DR Congo (Monuc).
The resolution was adopted as African countries involved in DR Congo postponed a summit originally scheduled to take place on Friday in Zambia.
Rwanda and Uganda support groups fighting the government of President Joseph Kabila, who in turn is backed by Angola, Namibia and Zimbabwe in the three-year conflict.
UN observers
The offensive against Moliro was the first major ceasefire violation in a year.
The RCD has offered to leave the town has long as the government promises not to retake it.
A number of UN observers arrived in the city on Monday.
The UN also called on Rwanda to exert its influence on the rebels to stop the fighting.
France has accused Rwanda of sending up to 10,000 troops to help the rebels seize Moliro.
Elections
The summit of African countries involved in the conflict, initially scheduled for Friday, has been put back to next Wednesday, 27 March.
The Congolese Government says the postponement was requested because President Kabila is due to visit China on Thursday.
Last week, the Kinshasa government pulled out of peace talks in South Africa in protest at the fighting in Moliro.
These talks - which opened officially last month - had involved representatives of the government, rebels and the political opposition.
They aimed to find a political solution to Congo's many years of instability.
Discussions were supposed to centre on a transition government which would oversee elections.