White House national security adviser Jake Sullivan said Sunday that the U.S. plans to "take additional strikes and additional action" following the deaths of three U.S. soldiers last week.

Why it matters: Sullivan's remarks come as the U.S. struck several Iran-linked targets in Syria and Iraq over the weekend in response to the deadly attack in Jordan.

What he's saying: "It began with the strikes on Friday night, but that is not the end of it," Sullivan said Sunday on NBC's "Meet the Press."

• Sullivan said they intend to take the additional strikes and action to "continue to send a clear message that the United States will respond when our forces are attacked or our people are killed."
• "At this point, we are still assessing the question of how many casualties there were among the militia groups," Sullivan said, adding that "we do believe that the strikes had good effect in degrading the capabilities of these militia groups to attack us."

Sullivan said they "do not have, at this time, any confirmation of any civilian casualties."

• He told CBS' "Face the Nation" on Sunday that the administration is "looking at the casualties" but that he does not "have anything to report ... this morning publicly on that."

Of note: Sullivan also told "Face the Nation" that he "would not describe" Friday's response as "some open-ended military campaign."

• "We have a concept of how we intend to respond. I'm not going to telegraph it on this show, but we will execute that concept with the kind of professionalism that only the U.S. military can bring to bear."

Zoom in: Biden told congressional leaders on Sunday that the recent military action is sanctioned in part by a broader 2001 measure that authorized the Global War on Terror and a 2002 Iraq War measure.

• The 2002 resolution, as well as the broader 2001 measure, have been the subject of fierce, bipartisan repeal efforts in recent years as lawmakers try to claw back war powers from the executive branch, Axios' Andrew Solender reports.

Asked whether the administration has ruled out strikes inside Iran, Sullivan said he is "not going to get into what we've ruled in and ruled out, from the point of view of military action."

• However, Sullivan noted that President Biden is "determined to respond forcefully to attacks on our people" and is also "not looking for a wider war in the Middle East."

Meanwhile, House Speaker Mike Johnson (R-La.) pushed back on Sullivan's remarks, telling "Meet the Press" on Sunday that "we need to make absolutely clear to Iran that nothing is off the table."

• "We should not be appeasing Iran," Johnson said, calling for "turning up the heat on Iran" and taking action to "decimate the Iran Central Bank."
• "There's a lot that we could do to Iran to send a message instead of this appeasement strategy," he said.
• Johnson added strikes inside Iran "should not be off the table."

Go deeper: U.S. launches retaliatory strikes on Iran-linked targets in Iraq and Syria

Editor's note: This story has been updated with additional comments from Sullivan and House Speaker Mike Johnson.