Secretary of State Marco Rubio said on Sunday that the United States remained open to diplomacy with Iran, even as U.S. forces intensified their military campaign with a ninth consecutive night of strikes in the country.

“We’ve tried multiple times with Iran, and we’ll continue to try. If that door opens, we’ll be happy to see it open,” Mr. Rubio told reporters at Joint Base Andrews in Maryland before he departed for an Association of Southeast Asian Nations summit in Manila.

However, he added, “Their behavior has to change in order for ours to change.”

Iran continued to send signals to the United States that it wanted to negotiate, both directly and through mediating countries, Mr. Rubio said, though he did not elaborate. There have been no public signs of diplomacy over the past week as the two sides escalated attacks, and no clear offramp to the fighting.

Mr. Rubio said of Iran: “Their behavior is what we’re responding to, and their behavior is they’re launching missiles and drones against ships, including tonight.”

Iran’s Islamic Revolutionary Guards Corps said on Monday that two oil tankers had “exploded and were forced to cease movement” after trying to traverse the Strait of Hormuz via a route that Tehran has warned ships against using. The Guards did not directly claim responsibility for the strikes. A British navy monitoring agency had said earlier Monday morning that a vessel was on fire off the coast of Oman.

Iran’s strikes on commercial vessels traversing the strait set off the back-and-forth attacks that have continued for more than a week. While the scope of the hostilities has since expanded, both Iran and the United States continue to assert authority over the waterway, a critical passage for global oil shipments.

Mr. Rubio said that Iran’s attacks on ships in the strait violated the terms of the memorandum of understanding signed between Washington and Tehran in June. Tehran has interpreted that deal to mean that it alone can determine which route ships take through the waterway, and has struck vessels that have sought to use an alternate route close to Oman’s coastline.

In response to a question about President Trump’s now-scrapped plan to charge a fee for guiding ships through the strait, Mr. Rubio called on other nations to step up and said it was “not the United States’ job to protect shipping for the entire planet forever.”

“Other countries need to begin to step up and provide — whether it’s hardware or finances — to help carry that burden,” he said.