NATO Secretary-General Jens Stoltenberg said on Monday that all members of the Atlantic alliance stood behind the United States in the Middle East, after a U.S. drone strike killed Iranian military commander Qassem Soleimani.

Speaking after a rare NATO meeting on Iran and Iraq in which the United States briefed its allies about last Friday's drone strike, Stoltenberg also called for a de-escalation of tensions, echoing the statements of some European leaders.

"We are united in condemning Iran's support of a variety of different terrorist groups," Stoltenberg said. "At the meeting today, allies called for restraint and de-escalation. A new conflict would be in no one's interest. So Iran must refrain from further violence and provocations."

Also Monday, United Nations Secretary-General Antonio Guterres warned that "geopolitical tensions are at their highest level this century." Guterres called on world leaders to stop escalating tensions, exercise maximum restraint and re-start dialogue.

Without naming any countries, Guterres told reporters: "This cauldron of tensions is leading more and more countries to take unpredicted decisions with unpredictable consequences and a profound risk of miscalculation."

On Saturday, NATO suspended training of Iraqi security and armed forces to ensure the safety of several hundred mission members in response to the killing of Soleimani.

"The safety of our personnel in Iraq is paramount," acting NATO spokesman Dylan White said in a statement. "We continue to take all precautions necessary. NATO's mission is continuing, but training activities are temporarily suspended."

He said NATO Stoltenberg had spoken by phone with U.S. Secretary of Defense Mark Esper since Friday's attack on Iranian Revolutionary Guards commander Qassem Soleimani at Baghdad airport.

NATO was monitoring the situation in the region very closely, he added, amid mounting concern that the killing of Iran's second-most powerful man could trigger a conflagration in the Middle East.