Defense Secretary Jim Mattis delivers a speech during a ceremony for the 70th anniversary of the Marshall Plan at the George C. Marshall European Center for Security Studies in Garmisch-Partenkirchen, Germany. (Christian Bruna/European Pressphoto Agency)

The Pentagon is putting the final touches on a promised new counter-Islamic State strategy for Syria and Iraq, and it looks very much like the one the Obama administration pursued, according to senior defense officials.

The core of the strategy is to deny territory to the militants and ultimately defeat them, and to stay out of Syria’s civil war pitting the Syrian government of President Bashar al-Assad, Iran and Russia against domestic opposition forces. The two fights in that country have come into increasingly close proximity in recent months, and there have been clashes.

Military officials from Defense Secretary Jim Mattis on down have emphasized in recent days that they are not looking for a fight with the regime or the Iranians. That has put them at odds with White House officials who have expressed concern about Iranian expansion across a new battlefield in Syria’s southern desert.

Critical of what they view as the Pentagon’s reluctance to prevent Iranian gains, these officials consider Iran’s increasing presence there a hindrance to the United States ’ pursuit of the Islamic State, and an attempt by Tehran to consolidate postwar control. Rather than allowing the regime and Iranian militia forces to plant their flags in the desert, the U.S. military and its proxy forces, they say, should be planting their own.

The differing views have emerged in recent weeks as the military and the administration have contemplated the next steps in Syria, once the U.S.-led coalition completes its ongoing campaign to eject the Islamic State from Raqqa, its de facto Syrian capital.

Even as the Raqqa offensive has gotten underway, regime and Iranian militia forces, having quelled the rebel uprising in much of Syria’s heavily populated western regions, have headed east across the desert with Russian air support. Their goals are the isolated, government-held city of Deir al-Zour, long surrounded by the Islamic State, and the town of Bukamal, along the Iraqi border.

The two are at opposite ends of Syria’s southern Euphrates River valley, an area the militants have long held. Much of their leadership has fled there from Raqqa, which lies farther north along the river.

Mattis agreed with a reporter Tuesday who asked if, rather than trying to prevent government and militia advances, the coalition is trying to “compartmentalize” the valley, effectively leaving the regime and Iran in control of areas they are able to conquer from the militants.

Asked whether he is worried about possible conflict with them as U.S. forces continue to pursue the Islamic State in the same area, Mattis said, “Not if the Iranian militia doesn’t attack us, no.”

The key, he said during a trip to Europe this week, is more precise deconfliction among commanders of various forces on the ground, and between the United States and Russia. Although the Russians claimed to have shut down lines of communication to protest the U.S. downing of a Syrian aircraft that came too close to American operations near Raqqa this month, several U.S. officials said those contacts have grown more robust and are operating effectively.

“It’s probably not going to look that neat,” Mattis said of the lines being drawn. “You know, it’ll be based on where does the river bend here and where is it — which side of the river is a town on there. . . . So it may look a little more squiggly.”

He added: “But as long as it’s worked out by the commanders and enough people know about it in sufficient times, there are ways that are proven that we can do this.”

In neighboring Iraq, coalition-supported Iraqi security forces are said to be close to victory in the long battle to retake the northwestern city of Mosul from the Islamic State. Once they do, defense officials said, at least some of them will move to the border area opposite Bukamal.

President Trump, who criticized former president Barack Obama’s strategy to defeat the Islamic State as weak, promised a new plan within 30 days of his inauguration and then turned it over to the military. Under congressional budget guidelines this year, lawmakers must review and approve the policy before funding for several aspects of it is released.

Several defense officials, speaking on the condition of anonymity to discuss the still-secret planning, said the strategy document is nearly ready to deliver to Capitol Hill. It is unclear whether Trump, or the Pentagon, will announce its outlines and goals.

H.R. McMaster, Trump’s national security adviser, spoke bluntly Wednesday of the destructive nature of Iranian influence in Syria and Iraq. “Iran is feeding this cycle of sectarian conflict to keep the Arab world perpetually weak,” he said at a conference held by the Center for a New American Security. He described Tehran as a source of much of the suffering in Syria, and said that 80 percent of the regime’s effective fighters were Iranian proxies.

But McMaster stopped well short of calling for American action to deny territory to Iran or the Syrian regime. “We could do a lot better to pull back the curtain on Iranian actions,” such as the hollowing out of Iraqi institutions or support for Syrian barrel bombs that have devastated civilian populations and infrastructure, he said.

“We have to be very clear that the reason we are in Syria is to destroy the ISIS,” McMaster said, using another name for the Islamic State. Outlining the administration’s approach to fighting the militants in Syria and Iraq, he said the key components are “denying them a safe haven and support base,” cutting off funding and discrediting the “perverted ideology” that they use to attract recruits to their cause.

The starkest statement of the military’s disinterest in confronting the regime and the Iranians came Friday when Col. Ryan Dillon, spokesman for the Baghdad-based coalition, said that if Assad’s forces “want to fight ISIS in Bukamal and they have the capacity to do so, then that would be welcomed.”

“We in the coalition are not in the land-grab business,” Dillon said. “We are in the killing-ISIS business. That is what we want to do, and if the Syrian regime wants to do that and they’re going to put forth a concerted effort and show they are doing just that in Bukamal or Deir al-Zour, or elsewhere, that means that we don’t have to do that in those places.”

Frederic C. Hof, a fellow at the Atlantic Council who was the Obama administration’s special adviser for Syria, called Dillon’s words an invitation to Iran and its “client regime . . . to take over whatever parts of eastern Syria they can grab.”

“The Trump administration correctly views Iranian domination of Syria . . . as contrary to American interests,” Hof wrote in a report Monday. Yet the policy now advocated by the U.S. military, he wrote, “will be good news indeed in Tehran and Damascus, to say nothing of Moscow.”

Thomas Gibbons-Neff, traveling with Defense Secretary Mattis, and Greg Jaffe in Washington contributed to this report.