WASHINGTON - The US-led coalition fighting the Islamic State group in Iraq and Syria on Friday acknowledged another 21 civilian deaths in past air strikes , bringing the toll up to 624.

In its monthly findings, the coalition said it had completed investigations into 132 allegations of civilian casualties.

“To date, based on information available, (the coalition) assesses that, it is more likely than not, at least 624 civilians have been unintentionally killed by coalition strikes” since the anti-IS campaign began in late 2014, the coalition said Friday.

Aside from probing civilian death reports that come in from coalition pilots and through social media and other channels, military investigators also continued wading through a huge backlog of hundreds of allegations reported by the website Airwars.org. The London-based collective of journalists and researchers has always had civilian death tolls that are wildly divergent from those acknowledged by the coalition. According to the most recent Airwars tally, 4,734 civilians have been killed in coalition strikes . Among the incidents the coalition said resulted in casualties was a May 12 strike near Raqa that targeted IS fighters but also killed 10 civilians in a nearby building.

“Although all feasible precautions were taken and the decision to strike complied with the law of armed conflict, unintended civilian casualties unfortunately occurred,” the statement read. As of the end of June, the military was still assessing 424 civilian casualty reports.

US approves sale of war planes to Nigeria to fight Boko Haram

The State Department has approved the sale of US warplanes to Nigeria to aid its fight against Boko Haram militants, ending a suspension of weapons sales that followed a deadly Nigerian military strike on a refugee camp.

The State Department informed Congress late Wednesday it had approved the $593 million sale of 12 Super Tucano A-29 ground attack aircraft, a deal that includes supplying the Nigerian armed forces with ammunition, training and aircraft maintenance, a US official said.

The US Air Force last year supplied such aircraft - bought from Brazilian plane manufacturer Embraer - to the Afghan military to help it fight the Taliban.

“These aircraft offer improved targeting capabilities, allowing Nigeria to more effectively lead the fight against Boko Haram and the ISIS West Africa branch, while also potentially reducing risks of collateral damage and civilian casualties,” said the US official, who asked not to be identified.

The training that the US intends to provide to Nigerian pilots “would help build the skills and procedures to effectively and responsibly operate the aircraft in accordance with international human rights law and the law of armed conflict,” the official said.

The sale was initially unveiled in May 2016 but the Democratic administration of former president Barack Obama froze the deal just before handing over the reins of power to President Donald Trump in January, after the Nigerian military accidentally bombed a camp for people displaced by the conflict in the northeast, killing 112 civilians.

Congress now has 30 days to decide on whether to approve the deal. If no opposition is voiced, the administration can go ahead with the arms sale.

Boko Haram, which has allied itself with the Islamic State, has been leading a bloody militant insurrection in Nigeria since 2009 that has left at least 20,000 people dead and forced another 2.6 million people from their homes.

Thousands of women and girls have been kidnapped and forced into marriage with Islamist insurgents or made to carry out suicide bomb attacks.

Last week, the military was ordered to strengthen its response to Boko Haram after 69 people were killed in an ambush.