Western and Arab attacks on Islamic State (Isis) in Iraq and Syria are halting the jihadis’ advances, but it could still take years to defeat them, the US warned on Wednesday as it emerged that Iran had also launched air strikes against the extremist group.

Officials in Tehran initially denied that it had done so. But an Iranian politician referred to a 25-mile “buffer zone” along the country’s border with Iraq where an operation had been conducted in cooperation with Iraqi forces. “We do not tolerate any threats within the buffer zone, and these targets were in the vicinity,” Hamid Reza Taraghi told the New York Times from Tehran. Dozens of Isis fighters had been killed in the strikes, the paper quoted him as saying.

John Kerry, the US secretary of state, told a meeting in Brussels of 60 countries taking part in the international coalition that the campaign had already had a “significant” impact on Isis. But Syria’s president, Bashar al-Assad, claimed the attacks had had little effect.

“Our commitment will most likely be measured in years,” Kerry told the meeting at Nato headquarters. Partners would “engage in this campaign for as long as it takes to prevail”, he pledged. The Isis “advance across Syria and into Iraq is being halted”, a communique said later.

“It is much harder now than when we started for Daesh [a pejorative Arabic acronym for Isis] to assemble forces in strength, to travel in convoys and to launch concerted attacks,” Kerry told reporters. “No large Daesh unit can move forward aggressively without worrying what will come down on it from the skies.” It had also been unable to gain control of the Syrian border town of Kobani from Kurdish rebels – a battle it looked poised to win just a few weeks ago.

US Central Command said in its latest communique on Wednesday that US forces had conducted 14 air strikes against Isis in Syria in the past three days and US and “partner nation military forces” had conducted 11 air strikes in Iraq in the same period.

The political complexities of the fight against the group were highlighted overnight when the Pentagon revealed that Iran’s air force had attacked Isis targets in Diyala province in eastern Iraq. Tehran is not a member of the coalition and officials quickly denied carrying out raids or acting in coordination with the US.

The US secretary of state declines to comment on reports that Iran has carried out attacks on Isis targets in Iran. Guardian

Kerry declined to comment on initial reports from Washington, but insisted: “Nothing has changed in our fundamental policy of not coordinating our military activity, or any other activity, at this moment with Iranians. We are not doing that.”

Rear Admiral John Kirby, the Pentagon spokesman, said earlier that the US continued to fly its own missions over Iraq and that it was up to the Baghdad government to avoid conflicts in its airspace.

A senior Iranian official said no raids had been carried out and Tehran had no intention of cooperating with Washington. “Iran has never been involved in any air strikes against Daesh targets in Iraq. Any cooperation in such strikes with America is also out of the question for Iran,” he said.

In Tehran, the deputy chief of staff of Iran’s armed forces, Brigadier-General Massoud Jazayeri said Iran considered the US responsible for Iraq’s “unrest and problems”. The US would “definitely not have a place in the future of that country”.

The exchanges followed reports that US-made F4 Phantom jets from the Iranian air force had been targeting Isis positions in Diyala. Jane’s Defence Weekly identified al-Jazeera TV footage of a jet flying over Iraq as an Iranian Phantom.

Not for the first time, the anti-Isis campaign has raised the intriguing possibility that the US and Iran, bitter enemies since the 1979 Islamic Revolution, might work together against a common foe. The model has been seen as their brief cooperation against al-Qaida in Afghanistan after the 9/11 attacks. Talks about Iraq have taken place in the margins of ongoing international negotiations about Iran’s nuclear programme. But the US has repeatedly denied coordinating with Iran.

Last month, following a letter sent by Barack Obama to Iran’s supreme leader, Ayatollah Seyed Ali Khamenei, the US national security adviser, Susan Rice, said that “we are in no way engaged in any coordination – military coordination – with Iran on countering Isil [another name for Isis]”.

The two countries also remain sharply at odds over the crisis in Syria, where Isis gathered momentum in the chaos of war and bloodshed before its sensational capture of the Iraqi city of Mosul in June.

The coalition fighting Isis includes the UK, France and Australia as well as Saudi Arabia, Jordan, the UAE and Bahrain – Sunni Arab states that are deeply suspicious of Iran’s regional ambitions and Shia identity.

The US-led air campaign against Isis began on 8 August in Iraq and was extended into Syria in September. But the UK and several other countries that operate in Iraq still refuse to do so in Syria – highlighting confusion about overall strategy towards the bloodiest crisis of the Arab spring, which has already cost around 200,000 lives and made millions of Syrians refugees.

Asked whether the US-led air strikes had been helpful to him – in targeting a dangerous enemy and weakening the ranks of his enemies – Assad told the French magazine Paris Match: “You can’t end terrorism with aerial strikes. Troops on the ground that know the land and can react are essential. That is why there haven’t been any tangible results in two months of strikes led by the coalition.”

The Brussels talks also looked at ways to stem the flow of foreign fighters joining Isis and how to counter its slick propaganda, disseminated on social media.