"It looks like a rather large-scale operation," Eliasson said about the attacks.
Jean-Marie Guehenno, UN undersecretary-general for peacekeeping operations, said a planned 26,000-strong peace force faced continued obstacles in deploying fully.
"For a peacekeeping force to operate in the midst of a war is a very dangerous position to be in," Guehenno said.
Amid ongoing conflict "a peacekeeping force cannot by and of itself stop that violence," he said.
Fighting in Ndjamena has fuelled mistrust
between Chad and Sudan [AFP]
Guehenno also said that mistrust between Sudan and its Chad risked widening the Darfur conflict beyond Sudan's borders.
"Continuing accusations by both governments ... increase the climate of mistrust, fuel tensions between the two countries, and once again demonstrate the potential for a conflict of international dimensions in the area," he said.
Chad's government has accused Khartoum of supporting an armed opposition alliance who launched an assault on Ndjamena last week in an attempt to force Idriss Deby, Chad’s president, from power.
The army later forced the opposition fighters from the capital.
Push for talks
The UN has been pushing Sudan's government and opposition Darfuri forces to hold fresh negotiations after a first round in Libya last October stalled.
Some opposition groups have signed a deal with the Sudanese government but others are still fighting.
"While the people of Darfur cannot wait for ever, we will have to accept that the steps toward an eventual peace agreement will be incremental and will take longer than we had initially hoped," Eliasson said.
International experts estimate that around 200,000 people have died and 2.5 million been displaced as a result of the conflict in Darfur.
The full deployment of the UN and the African Union joint peacekeeping force in Darfur has been delayed by disagreements with Khartoum. Only 9,000 troops are currently in place.
Thailand and Nepal have offered units to the UN-AU force but Khartoum has yet to accept them.