Speaking to journalists after a meeting of NATO defense ministers in Brussels at which US Defense Secretary James Mattis said other members of the alliance needed to contribute more to their own defense and meet NATO's target of defense spending of at least 2% of GDP from member states.
"The firm message from Secretary Mattis reflects the political reality in the United states," Stoltenberg said.
"I expect allies like Romania, Lithuania and Lavia to very soon reach the 2% targets," he said.
Latvia is due to meet the target next year.
But Stoltenberg also underlined that Article 5 is not conditional.
"Our collective defence clause, Article 5 is unconditional. It is absolute and is the core of the alliance," Stoltenberg said.
Describing Mattis as "a true friend of the alliance" who knew the organization better than many national leaders, Stoltenberg added:
"Because he is a true friend of the alliance he also conveyed a firm message about defense spending."
Policy towards Russia would be based on a "firm predictable approach" he said.
"And we have to combine dialogue with deterrence -- that is a lesson we learned during the Cold War."
The four multinational battlegroups being sent to Estonia, Latvia, Lithuania and Poland will be "fully operational" by the end of June, it was confirmed.