Stuttgart (Germany) (AFP) - US Defence Secretary Ash Carter warned Tuesday that NATO would defend its allies against Russian "aggression" as he presided over the passing of the alliance's European command to a new general.
Carter said NATO would "keep the door open for Russia" for cooperation on global security challenges if Moscow abandoned its "sabre-rattling".
"But it's up to the Kremlin to decide. We don't seek a cold, let alone hot, war with Russia. We don't seek to make Russia an enemy," he said.
"But make no mistake: we will defend our allies, the rule-based international order and the positive future it affords us."
Carter was speaking in the western German city of Stuttgart at the change of command ceremony for General Curtis Scaparrotti, commander of US forces in South Korea, to become its top commander in Europe.
Scaparrotti, who replaces US General Philip Breedlove, will have to handle the Atlantic alliance's tense relationship with Russia.
Breedlove, a US air force general, has served as Supreme Allied Commander Europe since May 2013. The post has always been held by an American.
Moscow has led a bombing campaign in support of President Bashar al-Assad in Syria and allegedly backed rebels in eastern Ukraine against forces of the Western-backed government in Kiev.
Since the Ukraine conflict, NATO has established a high-speed response force complete with forward command and logistic centres in its eastern members so it can deploy much more rapidly.
Last week Germany said it was considering sending troops to Lithuania as part of a NATO mission to reinforce the alliance's eastern flank with Russia.
In brief remarks, Scaparrotti said NATO faced "challenges to international law" by Russia in Ukraine and the Georgian breakaway region of South Ossetia.
"We also face the immediate threat that terrorism poses as the world witnessed in the recent tragedies in Brussels, Paris and Ankara," he said.
"And we face the significant influx of migrants and refugees that are challenging the social fabric of Europe."
NATO has an operation to stop migrant boats reaching the Greek islands from Turkey and member state Italy is pushing a plan to introduce NATO naval patrols off Libya in time for the summer, which is peak season for people smuggling.