The International Crisis Group said in a report that the African Union, monitoring a shaky ceasefire in Darfur, should also strengthen its mandate to give its forces powers to protect civilians and take preventive offensive action if needed.
"Bold new action is urgently required to safeguard the inhabitants of Darfur, many of whom are still dying, being raped or facing indefinite displacement from their homes," said the report by ICG, which has closely followed the Darfur conflict.
The report said a force of between 12,000-15,000 was needed within 60 days to stop violence against civilians in villages and more than 2 million in camps who have fled the fighting.
"If the AU cannot meet these objectives of time and personnel, NATO should help it do so by deploying its own bridging force that brings the total up to that level and maintains it there until the AU can perform the mission entirely with its own personnel," the report said.
Rebels in Darfur took up arms in early 2003 accusing the government of discrimination and neglect. They say Khartoum responded by backing Arab militias to drive non-Arabs from their villages in a campaign of killing, looting and rape. Tens of thousands of people have been killed in the conflict.
A 3,000-strong AU mission is monitoring the ceasefire, with limited powers to protect civilians if they witness an attack.
Sudan says it will only allow African forces to deploy in Darfur. NATO has said it would airlift extra African troops to the region, but has not said it would send its own forces.
The AU, which has received financial and logistical support from the EU and the NATO, plans to up its forces to more than 7,000 by October and to around 12,000 if needed by the beginning of next year.
A senior AU official told Reuters additional forces would have heavier weapons to protect themselves if attacked. In May, rebels detained and disarmed an AU team in Darfur, which raised questions over the effectiveness of the force.