What is the relationship between Jens Stoltenberg and NATO in the context of the input sentence. Choose an answer from: product/material produced; manufacturer; distributed by; industry; position held; original broadcaster; owned by; founded by; distribution format; headquarters location; stock exchange; currency; parent organization; chief executive officer; director/manager; owner of; operator; member of; employer; chairperson; platform; subsidiary; legal form; publisher; developer; brand; business division; location of formation; creator.
Text: PRAGUE, Sept 9 (Reuters) - Reports of growing Russian military activity in Syria are a cause for concern while the situation in eastern Ukraine seems calmer, NATO Secretary General Jens Stoltenberg said on Wednesday.
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