Iran live updates: Pentagon names soldier killed in Iraq by downed Iranian drone

President Donald Trump issued a warning to Iran on Monday following the deaths of three American servicemembers this weekend-- saying Iran "will pay" every time they kill Americans.

"Every time Iran kills an American Soldier they will pay for that killing many times over!" Trump wrote.

But the president did not elaborate on how Iran will "pay," or whether there will be consequences separate from the ongoing strikes that have hit the country the past nine consecutive nights.

Sunday night, Trump said the U.S. had struck Iran "in honor" of the fallen U.S. servicemembers.

In his post Monday, Trump added that "this directive has been passed on to Secretary of War, Pete Hegseth, Chairman of the Joint Chiefs of Staff, Daniel Caine, and every Leader in the Military"-- though it is unclear exactly what consequence to Iran is outlined in this alleged "directive."

--ABC News' Emily Chang and Michelle Stoddart

President Donald Trump will attend the dignified transfer ceremony of the fallen U.S. service members at Dover Air Force Base on Tuesday, White House press secretary Karoline Leavitt wrote on social media.

"President Trump will attend the dignified transfer of our fallen heroes tomorrow evening at Dover Air Force Base," Leavitt said.

"The President and the entire White House are praying for their families," Leavitt added.

-ABC News’ Michelle Stoddart

Secretary of State Marco Rubio told reporters that Iran is continuing to send messages "directly and through multiple channels with other countries," indicating it was willing to negotiate, and that the U.S. remains open to diplomacy -- but he noted that a "growing split" in Iran’s system complicates that.

"So, they’ll have to work out that split internally. We’re going to continue to respond," he said.

Rubio said the current military mission is solely focused on responding to Iran’s attacks in the Strait of Hormuz.

"The only reason why the U.S. is conducting strikes is against assets of Iran that are being used to attack global commercial shipping," Rubio said. "Other countries should join us in this effort, but the United States is involved not just in protecting those ships but in targeting the places that they use to launch these missiles and drones."

Rubio also spoke about the service members killed.

"The mission they carry out is very, very dangerous," he said. "Our hearts go out to the families of the heroes that paid the ultimate price."

-ABC News’ Shannon Kingston

President Donald Trump said the U.S. hit Iran "hard" in honor of the U.S. service members who were killed by Iran over the weekend.

U.S. Central Command said two service members were killed in an attack on Jordan on Friday, and that a third service member remained missing. Another service member was killed in Iraq on Saturday, CENTCOM said.

Trump said on Sunday the U.S. lost "great patriots," but said the mission of ending Iran's nuclear capabilities would carry on -- claiming that U.S. forces started another wave of attacks against Iran "in honor" of the service members lost.

"Well, we feel very badly, but you know those great people, those great patriots, were out there fighting that Iran cannot have a nuclear weapon. Iran has been very, very badly damaged," Trump told reporters after flying back from the World Cup final in New Jersey.

"We hit them very hard again tonight, and we did that in honor of the probably three -- It's probably three as opposed to two. Great patriots," Trump said.

-ABC News' Devin Garbitt and Isabella Murray