US officials claim Islamic State extremists have lost their momentum in fighting in Iraq and Syria and have been demoralised by heavy casualties inflicted by American air strikes.

The officials say air strikes since mid-November have killed senior and mid-level leaders as well as about 1,000 fighters, particularly around the fiercely contested Kurdish town of Kobani on the Syrian-Turkish border.

The most significant Isis figure to have been killed in recent strikes was identified as Haji Mutazz, also known as Abu Muslim al-Turkmani, a deputy to the movement’s leader, Abu Bakr al-Baghdadi. A Pentagon spokesman, Rear Admiral John Kirby, said the deaths had degraded its “command and control current operations”.

A senior US official said Isis had suffered particularly high casualties as a result of its determination to capture Kobani, sending many fighters to the border town, where they could easily be targeted by US planes. As a result, the official claimed, Isis fighters in the Isis Syrian stronghold of Raqqa were increasingly reluctant to go to Kobani and were growing disillusioned with the leadership.

“It was presenting itself as an unstoppable movement,” he said. “That kind of unstoppable momentum has been blunted, to say the least. We have killed over 1,000 of their fighters, particularly in Kobani, and that gets to Raqqa because they wanted to take Kobani and had been making the biggest flag they had ever made and they were going to put it up there because to them this is a war of flags.

“They were committed to this and we could see fighters flooding in from Raqqa and we saw that as an opportunity to attrit their manpower,” he said. “Fighters now don’t want to go to Kobani. There are similar accounts in Mosul, people not getting paid what they thought they’d get paid, not living the life they thought they’d promised … So the mood has changed.”

The official said it was too soon to tell whether the rate of attrition among Isis fighters had stemmed the flow of jihadi volunteers from abroad. The US is seeking to cut the flows from the home countries of the volunteers and at the main point of transit, the Turkish border.

The US is using Turkish bases for intelligence-gathering purposes but Ankara remains reluctant to allow its bases to be used for launching bombing sorties. However, Washington has persuaded the Turkish government to help train and equip moderate Syrian opposition groups, who would fight Isis as well as the regime of Bashar al-Assad. Turkey’s foreign minister, Mevlüt Çavuşoğlu, said on Fridaythat the training programme for Syrian opposition groups should begin before March.

At the same time, US advisers are operating alongside Iraqi trainers in Anbar province, helping train and organise Sunni tribesmen into regular units to confront Isis. The senior US official said several of the major Sunni tribes were determined to take on the extremist group after recent massacres of civilians, including women and children. About 250 tribesmen have been trained so far as part of the programme, which is an echo of the Anbar Awakening about eight years ago, in which Anbari tribes defeated the al-Qaida in the Iraqi movement that preceded Isis.

This time, the Iraqi government will seek to absorb the Sunni units into the national security forces.

Kuwait has agreed to defer next year’s Iraqi reparations payment dating to Saddam Hussein’s 1991 invasion, which would reduce Baghdad’s costs by $4.6bn, helping compensate for the slump in oil prices and allowing more to spent on building up loyalist forces to combat Isis.

But Sunni mobilisation is likely to face greater obstacles than during the Anbar Awakening. “This is going to take time,” the senior American official said. “The entire state structure has disintegrated in Anbar province. The tribal structure is not the same as when we were there … Even a prominent tribal sheikh can issue an edict and not everyone, not the teenagers, are going to listen.”