Maj. Gen. Robert Mood, whose three-month mission in Damascus ended last week, also said the situation in Syria is likely to remain unstable even if Assad's government steps down.
"It's impossible to imagine a future in Syria where the current people in power remain in power. So in that view, it's just a matter of time before this regime collapses. And that is how it's supposed to be," Mood told a news conference in Oslo. That could happen, he said, if more members of the military simultaneously leave the ranks of the government to join the opposition.
But Mood also said that if Assad's regime collapses, it might not be the end of Syria's many problems. "That might not be the start of a dialogue. That could easily be the start of a situation that is way worse," he said.
Since last week, Syrian rebels have made a run on the country's two largest cities, Aleppo and Damascus. Regime forces have responded with overwhelming firepower, ushering in some of the most serious violence the cities have seen in 17 months of conflict.
On Friday, U.N. Secretary-General Ban Ki-moon urged Syria not to launch an offensive against rebels in Aleppo amid reports of an army build-up in that city. "I urge the Syrian government to halt their offensive," Ban said at an Olympics event in London alongside British Foreign Secretary William Hague. "The violence from both sides must stop for the sake of the suffering civilians of Syria."
Assad "must call off this assault," said Hague, adding: "This utterly unacceptable escalation of the conflict could lead to a devastating loss of civilian life and a humanitarian disaster."
Mood warned that Assad's regime still has great military muscle left to apply in the conflict and that the situation could very well carry on for months or even years.
"The government has large military forces that still haven't been used to the full. Even if we in the past few weeks have seen combat helicopters and planes, there is still much (of that) left," he said, according to NTB, the Norwegian national news agency. "When it comes to violence, they have no sense of understanding for what we call proportionality."
In Morroco, Foreign Minister Saadeddine El Othmani said in an interview with The Associated Press on Friday that if Assad is removed, a transitional government must reflect Syria's diversity. He said the opposition groups fighting Assad are not unified, so that a transitional government "must unite the maximum number of tendencies ... to avoid civil war and tearing Syria apart."
The minister said Morocco will host the next Friends of Syria gathering of Western and Arab nations opposed to Assad, likely in early September. He wants Russia and China, which boycotted previous meetings, to be present. Their backing is needed in the U.N. Security Council to ramp up pressure on Assad.
AP correspondents Shawn Pogatchnik in London and Aziz El Yaakoubi in Morocco contributed to this report.