Civilian casualties have increased sharply in the US-led military campaign against the Islamic State in Syria and Iraq, with nearly 60% of the officially acknowledged deaths from the three-year war being reported in the first three months of the Trump administration.

US Central Command (Centcom) admitted to 484 civilian deaths up to the end of April as a result of coalition strikes as part of Operation Inherent Resolve, which began in August 2014. That compares with a cumulative total of 199 announced at the beginning of February.

The tallies are limited to those incidents that the US military has been able to investigate and confirm. The true death toll is likely to be much higher, as the battle to wrest control of densely populated west Mosul in Iraq from Isis continues, and the battle gets started for the Isis stronghold in Syria, in Raqqa.

Airwars, a UK-based watchdog group, estimates the civilian death toll from coalition airstrikes at over 3,800.

A Centcom spokesman said that the dramatic spike was largely caused by a single strike on 17 March when the bombing of a building in Mosul aimed at killing two Isis snipers called a building to collapse, killing 105 civilians. The spokesman also said that 80 previously undisclosed civilians deaths from earlier incidents had been added to the cumulative total in April.

However, human rights groups and other observers point to an array of other factors that suggest that civilian deaths from the counter-Isis campaign are likely to remain high and probably climb.

One of that factors is a legacy of the last weeks of the Obama administration, when targeting procedures were changed, removing the requirement for each sortie to be approved by a central “strike cell” in Baghdad.

That has meant that Iraqi forces fighting on the ground have been able to call in an air strike from a coalition member with planes in the area. It does not have be approved by the coalition as a whole.

The coalition also includes the UK, Netherlands, France, Canada, Australia, Denmark and Jordan. Saudi Arabia, Turkey and the United Arab Emirates also take part in air strikes in Syria.

The procedural changes in December, said Belkis Wille, a Human Rights Watch (HRW) researcher in Baghdad, “allowed for a quicker response time, but that also makes for more mistakes”.

HRW is calling for the old procedures or new equally rigorous guidelines, to be reinstated.

Another factor is the growing intensity of the fight for Mosul, where Iraqi forces supported by coalition air power are taking on Isis militants bottled up in the west of the city, home to 200,000 civilians whom Isis is using increasingly as human shields. But even as Isis fighters blend in with Mosul residents, the coalition is using bigger bombs and less accurate means of delivering them.

From an analysis 380 of bomb craters in west Mosul from fighting in March and April, HRW estimates that the coalition was now routinely dropping 500- and 1000lb bombs, much bigger warheads that the more precise ones used earlier in the campaign. Meanwhile, more mortars are being used by ground forces, and highly inaccurate improvised rockets are being fired by some Iraqi units.

A Central Command investigation into the 17 March airstrike that killed 105 civilians in Mosul was caused when a coalition bomb detonated an Isis arms cache and destroyed the whole building where many local residents were sheltering. However, observers pointed out that the bomb dropped on the building, a 500lb GBU-38 was far greater than necessary to kill two snipers.

The US defence secretary, James Mattis, has denied there has been any change to the rules of engagement used in the campaign against Isis. But Micah Zenko, a senior fellow at the Council on Foreign Relations said there is evidence that air strikes can now be called in by a colonel, rather than a one-star general, as was the case until this year.

“Those closer to the fight are more likely to call in lethal force and are less likely to follow a value-based approach,” Zenko said. He said that rhetoric coming from the leadership in Washington could also be having an effect.

“A change in the rules of engagement does not have to be a change in doctrine,” he said. “It can just be a change in tone and command climate. Mattis has again and again talked about an annihilation campaign, and that can an influence lower down.”