Three days after the United States unexpectedly launched air strikes against an al-Qaida cell in Syria, officials are offering varied and conflicting explanations on the precise nature of the threat posed by the group.

Hours after Tomahawk missiles slammed into buildings near Aleppo believed to be used by Khorasan, an obscure group said to be focused on exporting terrorism from Syria, the US military described it as involved in “imminent attack plotting” against western targets.

Yet it is unclear what Khorasan was planning, how far that planning advanced, and whether the US itself was a target. Nor is the US confident as yet that it has either killed Khorasan’s leaders or significantly degraded any threat Khorasan may pose.

US officials, few of whom would speak on the record, insist that the US acted on the best available intelligence and expressed confidence in it. For days, several have resisted elaboration, citing fear of jeopardizing intelligence sources.

Ben Rhodes, a deputy national security adviser and among Barack Obama’s most trusted foreign policy aides, told reporters on Tuesday that Khorasan was involved in “actual plotting that was ongoing from Syria”. Like other US officials, Rhodes left unclear if an attack itself was “imminent”, or if Khorasan’s planning for an attack was imminent.

A senior US official briefing reporters the day after the strike said that Khorasan was “nearing the execution phase” for an attack “in Europe or the homeland”. Hours before the strike, however, a different senior official had told the Guardian there was no indication of an imminent domestic threat from the group.

Yet on Thursday, the Pentagon’s chief spokesman dismissed questions about how mature any attack planning was, saying it was “near the end stages of planning an attack a western target”.

“I don’t know that we can pin that down to a day or a week or a month or six months. Doesn’t matter. Far better to be the left of a boom than the right of it,” said Rear Adm John Kirby, using military jargon about periods before and after an attack.

Leaks to the New York Times and the Washington Post cast doubt on the maturity of an attack emerging from Khorasan. Anonymous officials were cited as saying it was unclear if Khorasan had picked out targets, deployed operatives to execute them or otherwise set a specific plan in motion, with one describing Khorasan’s planning as “aspirational”.

Yet Ned Price, a National Security Council spokesman, said Thursday that the strikes against Khorasan “were not prophylactic measures”.

The question of imminence has legal implications. “The legal basis for attacking Khorasan in Syria is even more questionable, more in doubt than Isis,” said Mary Ellen O’Connell, an international law scholar at the University of Notre Dame.

O’Connell noted that the Justice Department has adopted a definition of imminence “as sometime in the future, which is not the legal or common-sense understanding of imminent” under international law.

A Democratic congressman on the House intelligence committee said that Khorasan was hatching an attack on passenger airliners after it had connected in some manner with al-Qaida’s Yemen affiliate, known by the shorthand of Aqap. Aqap operatives, or men trained by the group, have unsuccessfully attempted to detonate several airliners in recent years.

“It has certainly been many months that we have been briefed about the specific threat” from Khorasan, congressman Adam Schiff of California told MSNBC on Wednesday. “Teaming up with bomb makers in Aqap and Yemen and posing a very direct threat to our aircraft, to our homeland.”

Schiff said that the “most immediate threat to our country” does not come from the Isis – against whom the US has launched a wide-ranging air war in both Iraq and Syria, and against whom it plans to support proxy armies of Syrian rebels, Kurdish guerrillas and Iraqi soldiers – but “from groups like Khorasan”.

Even if Khorasan had progressed to a targeting phase, Kirby indicated on Thursday the US lacks visibility into what country Khorasan had in its cross-hairs.

“We don’t know whether it was in Europe or the US homeland, but we know they were getting close,” Kirby said.

It as well remains unclear what the strikes against Khorasan accomplished, either in terms of eliminating or degrading the threat from the group or killing its leadership.

“The assessment as to whether we’ve removed that threat or not is ongoing,” a spokesman for the US Central Command, Mark Blackington, said on Thursday.

“I can’t say with extreme confidence that we know we have in fact disrupted a specific attack, but we definitely know we have hit targets that belong to them, were of use to them,” said Kirby, who also could not confirm if the strikes killed key Khorasan leaders like Muhsin al-Fadhli, a 31-year-old said to have been an al-Qaida member since his teenage years.

Robert McFadden, a former special agent with the Naval Criminal Investigative Service, said the strike may have had “a chilling or shock effect” on Khorasan, “even if they didn’t whack all the key leaders”.

The new Iraqi prime minister, Haider al-Abadi, said at the United Nations on Thursday that Isis was plotting attacks on subways in New York and Paris, but several US security sources cast doubt on the veracity of the threat. The day before, the governors of New York and New Jersey, Andrew Cuomo and Chris Christie, said they would step up security at major transit points, to include train stations, bridges and tunnels.

The Department of Homeland Security ramped up aviation security this summer, something spokeswoman Marsha Catron indicated was in response to Khorasan. Catron indicated the agency will remain on alert for a persistent threat from the organization.

“DHS, in conjunction with our interagency partners, will continue to adjust security measures, as appropriate, to protect the American people,” Catron said.