Cameron said the alliance, set up in 1949 to protect Western Europe from the Soviet Union, had to show that its Article 5 blanket security guarantee was still valid for all 28 members.
"As Russia tramples illegally over Ukraine, we must reassure our members that we will always uphold our Article 5 obligations," Cameron told the second day of a NATO summit.
To do that, NATO "must be able to act more quickly," he said, hoping alliance leaders would agree today to set up a new very rapid reaction force headquartered in Poland which could be deployed "anywhere in the world in just 2-5 days."
The two-day NATO summit has been billed as the most important since the end of the Cold War.
The upgraded rapid reaction force is meant to both reassure former Soviet satellites such as Poland and the Baltic states who have been rattled by Russia's actions in Ukraine and to show the alliance is measuring up to new threats, especially in a volatile Middle East.