The US-led coalition against the Islamic State group said Wednesday it was finally stopping the advance of the jihadists across Iraq and Syria, but warned it could take years to finally defeat them.

Washington meanwhile said that arch-foe Iran had launched its first air strikes against the Sunni extremist organisation, but denied any military coordination with Tehran, which is not part of the coalition.

But in a sign of the deepening complexity of the regional conflagration, Syria's Iranian-backed President Bashar al-Assad criticised the Western and Arab air strikes for having no effect.

US Secretary of State John Kerry told a meeting in Brussels of officials from 60 nations in the coalition of Western and Arab states that a campaign of around 1,000 strikes had had a "significant" impact on IS.

"Our commitment will most likely be measured in years," he told the meeting at NATO headquarters in the Belgian capital, adding that the partners would "engage in this campaign for as long as it takes to prevail".

The coalition later issued a closing statement saying that the militant group's "advance across Syria and into Iraq is being halted", and that Iraqi and Kurdish forces were reclaiming territory.

They also agreed at the Brussels meeting to develop a "multifaceted" strategy to combat IS, including stopping the flow of foreign fighters, cutting finance and "delegitimization" of its powerful, social media-driven brand.

- Assad criticises air strikes -

The United States launched its first strikes against IS in Iraq in August, two months after IS proclaimed a caliphate in Iraq and Syria. In late September the strikes were extended to IS targets in Syria, involving the United States as well as a number of allies.

Saudi Arabia, the United Arab Emirates, Jordan and Bahrain are taking part in the air strikes in Syria. Australia, Belgium, Britain, Canada, Denmark, France and the Netherlands are participating in Iraq.

The US has carried out the vast majority of the strikes against the group -- which is estimated to number around 30,000 jihadists, and is accused of atrocities including rape, crucifixion and the beheading of Western hostages.

The fight against IS has distracted from Western efforts to end Syria's brutal civil war, which has killed more than 195,000 people since it began as an anti-Assad revolt in 2011.

Assad, whose main backers are Tehran and Moscow, hit out at the Western powers that had until months ago been focused on his removal from power.

"You can't end terrorism with aerial strikes. Troops on the ground that know the land and can react are essential," he said in this week's edition of French magazine Paris Match.

But the Syrian conflict has created a constantly shifting patchwork of regional alliances, the most unlikely being that of Washington and Tehran, as underscored by the reports of Iranian airstrikes against IS.

The Pentagon said earlier that antiquated Iranian F-4 Phantom jets -- acquired from the United States before the 1979 Islamic revolution -- deployed against IS fighters in eastern Iraq eastern province of Diyala, after video emerged of the attack.

"We have indications that they did indeed fly air strikes with F-4 Phantoms in the past several days," Pentagon spokesman Rear Admiral John Kirby told AFP.

- US-Iran detente? -

Iranian forces have been active on the ground in Iraq assisting Shiite militia and Baghdad government units, but this was the first time the United States had confirmed the Iranian air force was taking part.

Tehran refused to confirm or deny the air strikes.

"There has been no change to Iran's policy to provide support and advice to Iraqi officials in the fight against (IS)," foreign ministry spokeswoman Marzieh Afkham said.

The alleged air strikes suggested a tacit understanding between mainly Shia Iran and the United States to tackle the common threat from the Sunni extremist group on Iran's borders.

The fight against the IS comes amid a US diplomatic drive to agree a deal with Iran over its nuclear program, and officials acknowledge the two sides have discussed the war in Iraq on the margins of the nuclear talks.

US President Barack Obama was reported earlier this month to have secretly written to Iran's supreme leader Ayatollah Ali Khamenei to discuss possible cooperation against IS providing there is a nuclear deal.

Iran and the US have been estranged since Iranian students took dozens of Americans hostage after the revolution that toppled the US-backed shah.

The conflict has also spilled over into Lebanon, which said on Tuesday it had arrested a wife and son of Abu Bakr al-Baghdadi, the self-proclaimed caliph of IS.

Meanwhile the head of Egypt's Al-Azhar, one of the most prestigious centres of Sunni Islam learning, on Wednesday condemned "barbaric crimes" committed by the Islamic State group.