New Acting U.S. Secretary of Defense Mark Esper called on NATO allies to denounce and combat the Iranian threat.

“I discussed the need to internationalize this issue by encouraging NATO allies and regional partners to voice their opposition to Iran’s bad behavior and to help us deter further provocative acts by improving maritime security,” Esper told reporters on Thursday following closed-door discussions at the organization’s headquarters in Brussels.

“The department’s mission remains clear: to deter conflict and, if necessary, fight and win on the battlefield,” he added.

Esper’s visit to NATO and his remarks came as tensions between the United States and Iran have increased of late.

Previously U.S. Secretary of the Army, Esper replaced Patrick Shanahan on Monday as acting Secretary of Defense after Shanahan withdrew his nomination last week to serve in the position permanently. The decision came after USA Today published an explosive report about alleged domestic violence between him and his former wife, Kimberly Johnson.

Esper is expected to be nominated to the top Pentagon role permanently.

Last week, Iran shot down a U.S. drone—a RQ-4A Global Hawk that was flying in international airspace above the Strait of Hormuz, though Iran claimed it was flying above its own airspace. In doing so, Iran’s Revolutionary Guards Corps’ (IRGC) top commander, Gen. Hossein Salami, said his country is “completely ready for war.”

U.S. President Donald Trump called off retaliatory strikes against Iran for that action, saying that such a response was “not proportionate to shooting down an unmanned drone.”

Still, the United States conducted a cyber attack against Iran last week.

Iranian President Hassan Rouhani said Tuesday that the Trump administration is “suffering from mental disability” and acting as “no sane person.”

Trump responded to Rouhani, tweeting “that any attack by Iran on anything American will be met with great and overwhelming force.”

On Sunday, Iranian-allied Yemeni Houthi rebels killed one person and wounded seven in an attack on a Saudi Arabian airport.

The United States has sent troops numbering in the thousands amid the threats emanating from Tehran.