Acting U.S. defense secretary Mark Esper said he isn't seeking armed conflict with Iran and encouraged U.S. allies to publicly denounce what he called Iran's "malign activities." His comments came at a meeting of defense ministers from the 29-member NATO alliance at its headquarters in Brussels.  

The conflict between Washington and Tehran has escalated in recent weeks following attacks on oil tankers in the Persian Gulf, which the U.S. blames on Iran, as well as the downing of an American military drone, which Iran says violated its airspace.  

Iran has denied responsibility for the oil tanker attacks while the U.S. says its drone was flying in international airspace. 

Esper, who just started in the job this week, called on NATO allies to help move tensions with Iran from a "military track" to a "diplomatic track." 

"Our goal is to bring Iran to the negotiating table to conclude a comprehensive, enduring deal that addresses Iran's nuclear program, its ballistic missile development and proliferation, its support for terrorism and other malign activities," Esper said.  

However, Esper cautioned that the U.S. would be ready to act if necessary. 

"Again, we do not seek armed conflict with Iran, but we are ready to defend U.S. forces and interests in the region. No one should mistake restraint for weakness," he added. 

NATO Secretary General Jens Stoltenberg echoed Esper's concerns, although he didn't announce any concrete steps from the alliance. 

"All allies share concerns when it comes to Iran's destabilizing activities in the region, their support for different terrorist groups, the missile program of Iran and also the announcement that they will now start to enrich uranium again," Stoltenberg said. 

Iran has said it plans to begin enriching uranium beyond levels agreed to as part of the nuclear agreement reached in 2015 with the U.S., China, Russia, Germany, France, the UK and the European Union in exchange for lifting sanctions. The U.S. withdrew from the deal last year, despite the objections of the other signees, who are still complying with the agreement for now.  

In addition to Iran, which wasn't on the official agenda, the defense ministers' meeting also focused on planning NATO's response to the possible end of the INF treaty, signed between the U.S. and the U.S.S.R. in 1987 banning nuclear-capable short and medium-range missile in Europe. NATO says Russia's new SSC-8 missile system violates the treaty and has urged Moscow to destroy the weapons by August 2 as the deadline. 

Russia insists that the missiles are compliant with the treaty. The U.S. has already said it would withdraw from the treaty unless Russia comes into compliance. 

Stoltenberg said NATO has no plans to deploy nuclear weapons in response but did say that they are looking at steps such as using conventional weapons as a deterrent and increased surveillance and air defense, if Russia doesn't get rid of the weapons. 

Other topics covered at the two-day meeting were NATO's ongoing mission in Afghanistan, the global battle against ISIL, and the alliance's new space strategy.