Civilian deaths are surging at an astonishing rate in the US-led fight against ISIS, a tangible sign that the war is growing more dangerous, not less, as Washington and its allies steadily regain territory that had been lost to the group.

In part, it reflects a major shift in the fight’s dynamics. Iraqi, Kurdish, and US forces are no longer battling ISIS in remote and sparsely populated parts of the two countries. Instead, the battle has moved to densely-packed cities like Mosul, where the large civilian populations mean that a single errant airstrike can cause heavy casualties.

And that, tragically, is what appears to be happening. Residents of the Iraqi city of Mosul report that as many as 200 civilians have been killed in airstrikes in their city in recent weeks, including one strike on March 17 which may have taken the lives of more than 100 people in a single blow. If they’re confirmed, they would mark some of the biggest death tolls from airstrikes in Iraq since the US launched an invasion of the country in 2003. The US military is now investigating the incidents.

Syria, too, has been hit by US airstrikes with some remarkable civilian casualties this month. A US strike in a rural area of Raqqa province killed up to 30 noncombatants who had taken shelter in a school last week, according to residents’ reports. And the week prior, the Syrian Observatory for Human Rights claimed that 42 people, most of whom were civilians, were killed by a US bombing in the town of Al Jinah, in what it deemed a “massacre.” The US military said it had a legitimate military target in the area, but noted it would investigate possible civilian loss of life.

The uptick in civilian deaths has been so sharp that it’s overwhelmed Airwars.org, a nonprofit that tracks civilian deaths from airstrikes in the Middle East. The site has had to scale back its monitoring of Russian airstrikes in Syria and focus instead on bombings carried out by the US and its allies.

“Almost 1,000 civilian non-combatant deaths have already been alleged from coalition actions across Iraq and Syria in March — a record claim,” Airwars said in a statement. “These reported casualty levels are comparable with some of the worst periods of Russian activity in Syria.”

US military officials have said they’ve been working hard to avoid civilian casualties. And, crucially, they’ve also said that there have not yet been any changes to the rules of engagement — protocols regarding the use of force — in Iraq and Syria, as Trump suggested he would do on the campaign trail. A spokesperson for the US Central Command said on Monday that the head of the command, Gen. Joseph Votel, “is not looking into changing the way we operate, other than to say our processes are good and we want to make sure we live by those processes.”

So what’s the explanation? There’s a number of possible factors, but from what we can see right now have less to do with any observable policy change than they do with the fact that the nature of the fight against ISIS is changing. The easier parts of the war against the radical Islamist group, which has mainly involved targeted airstrikes in less densely populated areas, are no longer the main focus. ISIS’s main urban strongholds are now in the crosshairs, and we’re going to see more American boots on the ground to help take them. If you’re a civilian in these cities, you’re in a very perilous situation.

Trump’s airstrike policy doesn’t look all that different from Obama’s yet

US military officials have said that airstrikes are ramping up as the fight against ISIS intensifies in the region. That escalation of strikes, however, doesn’t in and of itself seem to be an explanation for the sharp increase in civilian casualties.

Michael O’Hanlon, a senior fellow at the Brookings Institution who specializes in US defense strategy, said that current air campaigns reflect “more continuity than change” from the Obama administration, and that the increase in strikes appears in part to be “the natural ebb and flow of battle, not Trump policy.”

That seems to accord with data from the US-led coalition against ISIS, as you can see in this chart put together by Airwars below. January and February are markedly more intense than the immediately preceding months, but not a total departure from heavier months of strikes during the Obama administration, which began its campaign against ISIS in 2014.

The most notable feature of the chart is the last bar all the way to the right, representing strikes in March up until the 24th. It’s possible that when the chart is updated at the end of the month that it will look more on trend with the first two months of Trump’s time in office. But as of now, it looks like March may end up being a relatively light month in terms of number of airstrikes.

And yet it’s been an absolutely brutal one in terms of claims of civilian deaths. Take a look at the bar furthest to the right below to get a sense of the disconnect between volume of strikes and civilian deaths.

So what exactly is happening? The fog of war doesn’t allow for easy answers, and we’re too early into the Trump administration to render definitive judgments about what kind of effect the new president is having on the handling of the war. But one set of plausible explanations revolve around the basic fact that the nature of the war against ISIS is intensifying, and changing in a manner that’s more likely to take the lives of civilians.

“The US is moving more into environments that look like conventional warfare, moving up the spectrum from targeting individuals to participating in ongoing conflict, and that’s going to produce more casualties,” said Heather Hurlburt, a policy director at the New American Foundation and a former State department official.

ISIS has lost about a quarter of the land it controlled in Iraq and Syria in the past year. At its peak, it controlled about 40 percent of Iraqi territory; now it controls about 10 percent. But a great deal of that combat was in more rural and less densely populated areas. Now US-led forces are focusing on urban areas, and the fight to seize western Mosul, a crucial ISIS stronghold, is underway.

“It has been clear all along that the fight for Mosul was going to get harder and harder the further in you got,” Hurlburt says. “This is exactly what everybody expected.”

Still, the sheer size of some of the civilian tolls suggest there could be other factors at play. “We’ve dealt with ISIS in urban contexts before and typically we don’t have these massive civilian casualty events in urban warfare,” says Oona Hathaway, an international law professor at Yale University and a former national security lawyer in the Defense Department’s Office of General Counsel.

One possible explanation she points to is the that increased US troops on the ground could make commanders more tolerant of civilian casualties as they make decisions designed to protect growing numbers of embedded US military personnel. In March, the president sent 400 more troops to Syria, which included a team of Army Rangers and a Marine artillery unit, almost doubling their presence in the country.

That trend of troop increases is likely to continue if Trump stays true to his word about ramping up the fight against ISIS. US military officials announced on Monday that the US is sending an additional 240 soldiers to Iraq as part of the effort to retake western Mosul. They aren’t expected to participate in front-line combat, but they will be in dangerous environments on their operation.

Both Hurlburt and Hathaway believe that these kinds of increases in the number of military personnel on the ground may change the kind of calculus about use of force that commanders make as they balance duties to leave civilians unharmed against duties to protect their own troops.

It’s difficult to pinpoint patterns in the cause of civilian deaths over so short a period of time, and there could be other explanations that emerge for why they’ve spiked so sharply. The only thing that’s clear at the moment is that the war against ISIS is costly for innocents.