Speaking at a press conference in Bucharest, the Romanian capital, Stoltenberg said NATO was following European Union efforts "very closely" and that a solution to Greece's economic problems would be good for Greece, the EU and NATO.
One of the issues that divided Greece and its creditors before talks broke off last week was defense spending.
The Greek government had offered to cut 200 million euros ($222 million) annually from its defense budget but lenders demanded twice that amount.
And the International Monetary Fund is putting the blame for Greece's current economic predicament largely on the Greek government of Prime Minister Alexis Tsipras.
It noted, for example, that Greece has been slow to privatize state assets.
In 2011, the IMF predicted that Greece would raise 50 billion euros from selling off state properties by the end of 2015; so far, it has only raised 3.2 billion euros through privatizations.
The multinational lending agency says Greece will need debt relief and 50 billion euros ($56 billion) from October through 2018.
It says European creditors will have to come up with 36 billion euros ($40 billion) of that new financing and the credit will have to be offered on "highly concessional terms" — low interest rates and long repayment periods.