Secretary of State Mike Pompeo accused Iran of attacking a pair of oil tankers in the Gulf of Oman on Thursday as part of a “unacceptable campaign of escalating tension” by its leaders, but said the United States won’t respond with military force for now.

“This is only the latest in a series of attacks instigated by the Islamic Republic of Iran and its surrogates against America and its allied interests,” Pompeo said in brief public statement hours after the attacks. “They should be understood in the context for 40 years of unprovoked aggression against freedom loving nations.

He labeled the actions “a clear threat to international peace and security and a blatant assault on the freedom of navigation.”

But Pompeo said the American response to the hostilities would remain “an economic and diplomatic effort to bring Iran back to the negotiating table.” Failing that, “the United States will defend its forces, interests and stand with our partners and allies to safeguard global commerce and regional stability.”

The two vessels were traveling from the Persian Gulf on June 12 before being attacked in the gulf, just outside the Strait of Hormuz. The incident occurred in sea lanes that U.S. officials say have been heavily mined by the Iranian military.

U.S. Naval Forces responded to distress calls from the ships early Thursday morning. The guided missile destroyer USS Bainbridge served as the command vessel evacuation and assistance operations.

At least 21 crew members from the oil tanker Kokuka Courageous were taken aboard the Bainbridge following an explosion, Central Command officials said in a statement.

Shortly after news of the attacks broke, Senate Majority Leader Mitch McConnell, R-Kent., the attacks “threaten the very underpinnings of the global trading system” and freedom of movement in international waterways.

“What is clear is the growing tension and instability out in that region, to U.S. personnel, interests, and partners posed by Iran,” he said.

Pompeo said his department is confident of Iran’s culpability due to “our intelligence, the weapons used, the level of expertise needed to execute the operation, recent similar Iranian attacks on shipping, and the fact that no proxy group operating in the area has the resources and proficiency to act with such a high degree of sophistication.” He provided no additional evidence.

Critics of President Donald Trump have accused his administration of rushing towards a war with Iran, and escalating tensions with hyperbolic language and accusations.

Hours before the attacks, during a House Armed Services Committee mark-up, a group of Democrats unsuccessfully tried to add language the annual defense authorization bill draft restricting the president’s ability to declare war on Iran.

“I fought against Iranians when I was in Iraq. If necessary, I’d do so again. But right now, going to war with Iran is not necessary,” said Rep. Seth Moulton, D-Mass. who is running for president. (Members of) the Trump administration are trying to drag us into Iran just as they dragged us into Iraq.”

Iranian officials have denied any involvement in the most recent or past attacks on tankers in the region.