NATO on Monday welcomed a bailout deal reached between its "staunch ally" Greece and EU leaders, saying the nation's economic stability was also vital for the "security" of other NATO countries.

"I welcome the agreement because I think that it is important for the Greek economy but also for the whole of Europe and NATO," NATO Secretary General Jens Stoltenberg said during a visit to Slovenia.

"The stability, economic prosperity (of Greece) is important for many reasons, but of course also important for the security of all our countries."

Stoltenberg described Greece as a "committed ally".

"It's an ally that has spent more than two percent of GDP on defence for several years and I'm absolutely confident that Greece will continue to be a staunch and committed ally," he said.

"Greece is going to invest in defence also in the future."

Greek Prime Minister Alexis Tsipras agreed on Monday to tough reforms in return for a three-year bailout worth up to 86 billion euros ($96 billion), the country's third rescue programme in five years.