The Pentagon has admitted it did not kill a senior Islamic State operative in a March airstrike that the Obama administration made a talking point for success in the two-year war in Iraq and Syria.

Pentagon spokesman Peter Cook told reporters on Thursday that Tarkhan Tayumurazovich Batirashvili, also known as Abu Omar al-Shishani or Omar the Chechen, is now believed to have attended a 10 July meeting of Isis officials near Mosul, the jihadist army’s Iraqi capital, that was targeted in a US airstrike.

Cook said he was “not able to confirm” that Shishani was killed this time, although on Wednesday Isis announced through its propaganda agency that Shishani was dead.

“Indications [are] he was present” at the targeted 10 July meeting, Cook said, adding that earlier intelligence “led us to believe he had been killed” in March.

“We believed the assessment in March was correct, and received information that he was still alive,” Cook said, characterizing this as “very recent information” and denying that the Pentagon had intentionally avoided walking back its high-profile March assessment.

On 11 July, the military’s daily announcements of airstrikes mentioned two strikes the previous day near Mosul. Neither strike was said to have been aimed at an Isis meeting. Using the government’s preferred shorthand for Isis, the official statement referred to the strikes near Mosul hitting, among other targets, “five Isil assembly areas”.

After the Pentagon said it had targeted Shishani in a 4 March airstrike in Syria, numerous administration and military officials stated unequivocally the US had killed a man it described as an Isis minister of war.

On 17 March Ashton Carter, the secretary of defense, testified to a Senate panel that “we also recently killed Isil’s minister of war, the Chechen fighter Omar al-Shishani”.

On 30 March, Brett McGurk, a senior state department official with portfolio for the war against Isis, boasted to CNN that “it’s not a surprise that we were able to target and kill the overall military emir of Isis, Omar Shishani, just south of Shadadi”.

In an April briefing discussing a different Isis member allegedly killed in a US airstrike, Colonel Steve Warren, the former spokesman for the war against Isis, called the man “a known associate of Omar Shishani, who was Isil’s minister of war, who as you know we killed on 4 March”.

The following week, Warren said Shishani’s death, along with that of other Isis leaders, had dealt a “significant blow to the organization”.

On Thursday, Cook denied that the Pentagon had intentionally withheld information indicating that the US had not actually killed Shishani in March.

“The intelligence assessment made at the time was he had been killed, and that’s the understanding we had been operating under,” Cook said, until an unspecified but “recent” time.

Yet Cook also insisted that the suspected presence of Shishani near Mosul on 10 July was an indicator of the “importance” of a long-telegraphed and often-delayed impending offensive on Iraq’s second city to reclaim it from Isis.

A newly announced reinforcement of 560 US troops to a recaptured airfield near the city, to be used as a staging ground for the effort, will arrive “relatively quickly”, Cook said.