He also asked that the Sudanese government to provide a written and detailed agreement for more than 10,000 international troops to deploy as part of a hybrid force of UN and African Union peacekeepers.
While Natsios declined to comment on what Washington might do if Sudan fails to act by the year's end, he said the US and others are considering options that varied from travel bans on Sudanese officials to an assets freeze and imposing a no-fly zone in Darfur.
Frustrations
Natsios said Sudan could not "cherry-pick" the plan - accepting some elements but rejecting others - because nations would only contribute troops if Khartoum agreed to a contingent large enough to ensure their own self-protection.
The US official said he had been unable to visit Darfur on his recent trip to Sudan, because "the province is in such trouble now in terms of violence, instability and chaos, I couldn't get into the airports".
"Most of the airports were closed because there is so much fighting," he said.
US officials have voiced growing frustration at Sudan's refusal to allow international troops to go to Darfur, where more than 200,000 people have been killed in three years of fighting that the United States says is genocide.