(CNN) Investigations are continuing Friday into a "misdirected" US-led coalition airstrike that killed 18 allies in Syria earlier this week.

The strike, carried out Tuesday near the ISIS stronghold of Raqqa, killed members of the Syrian Democratic Forces (SDF), which the US and other nations have backed and coordinated with to fight ISIS militants in the country.

The latest deaths come amid mounting concern over the civilian toll of coalition airstrikes, as efforts to drive ISIS from Raqqa, its de facto Syrian capital, and Mosul in Iraq intensify, and in the wake of last week's US missile strike against a Syrian government target.

SDF General Command's senior military adviser, Nasser Haj Mansour, told CNN that members of the SDF had coordinated the airstrike, based on inaccurate information.

He said there was an ongoing investigation to find out the circumstances of the accident.

"We have no doubt on the coalition's support and capability, and there is a high level of coordination with our friends in the coalition," he added.

Mansour said 17 SDF members had been killed in the strike south of Tabqa, a strategically important town south of Raqqa -- one fewer than the number given by US Central Command.

'Tragic incident'

US Central Command acknowledged the strike in a statement Thursday and said the coalition was assessing the incident.

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"The strike was requested by the partnered forces, who had identified the target location as an ISIS fighting position. The target location was actually a forward Syrian Democratic Forces fighting position," it said.

"The coalition's deepest condolences go out to the members of the SDF and their families. The coalition is in close contact with our SDF partners who have expressed a strong desire to remain focused on the fight against ISIS despite this tragic incident."

A coalition official told CNN the SDF had asked for the coalition to perform the strike. The official said the coalition did not realize the error and was investigating how the mistake occurred and how it could be prevented in the future.

The airstrike was carried out in a key area for the coalition as allied forces on the ground close in on Raqqa, held by ISIS since early 2014.

It's not the first time the US-led coalition has been involved in a friendly fire incident involving allied forces in Iraq and Syria.

The Iraqi military said its forces suffered casualties during coalition airstrikes on ISIS positions near the Iraqi city of Falluja in late 2015.

Lavrov: Russia, Syria, Iran 'united'

The Tabqa airstrike came only days after US forces struck the Shayrat Syrian airbase with 59 cruise missiles, launched in retaliation for an alleged chemical attack by Syrian regime forces in the rebel-held town of Khan Sheikhoun on April 4.

Russian Foreign Minister Sergey Lavrov, speaking Friday alongside his Iranian and Syrian counterparts in Moscow, said the strikes were a failed effort to undermine the peace process in Syria and to change the regime. "Attempts of this kind will never be a success. It will never happen," he said.

He said the three countries were united in condemning the US missile strikes. "We demand that the United States should respect the sovereignty of a state and avoid such actions that threaten the current world order," he said.

Lavrov also criticized the Organization for the Prohibition of Chemical Weapons, saying he doesn't trust the two investigations the international watchdog is conducting into the Khan Sheikhoun incident -- probing the respective government and opposition claims -- because they are headed by the British, which he suggested showed "bias."

He called instead for an investigation involving representatives of all the UN Security Council members, which include the US and Russia.

Russia, a close ally of Syria, on Wednesday vetoed a UN Security Council resolution which sought to condemn the killings and call on the Syrian regime to cooperate with an international investigation into the attack.

In an AFP interview Wednesday, Syrian President Bashar al-Assad dismissed reports of the chemical attack as "100% fabrication" -- an assertion at odds with numerous witness accounts and expert analysis -- and claimed they were intended to give a pretext for the US strike.

Civilian casualties

The US-led coalition issued a report Saturday that said at least 229 civilians likely have been killed by coalition strikes in Iraq and in Syria since Operation Inherent Resolve began nearly three years ago.

The number could grow significantly if an investigation into airstrikes on a neighborhood in west Mosul on March 17 finds the operation unintentionally killed scores of residents there.

The top US commander in Iraq, Gen. Stephen Townsend, said last month that there was "a fair chance" a US strike had been involved.

The head of Iraqi civil forces working at the Mosul site told CNN that 141 bodies have been recovered.