MURSITPINAR, TURKEY (AFP) - US-led air strikes in Syria were reported Thursday to have killed more than 500 militants in a month, as Kurdish fighters prepared to reinforce the embattled border town of Kobane.

The battle for Kobane has become crucial for both the Islamic State (ISIS) group and its opponents, with a senior US official saying the Kurds there were inflicting heavy losses on the militants.

ISIS, which declared in June a "caliphate" over territory it seized in Iraq and Syria, was described Thursday as the world's wealthiest "terror" group, earning US$1 million a day from oil sales alone.

The Kurds in Kobane have been holding out against it for more than a month, buoyed by a promise of Iraqi Kurd reinforcements and by US air drops of weapons.

Turkish President Recep Tayyip Erdogan said Thursday that 200 Iraqi Kurd peshmerga fighters would travel through his country to join the battle in Kobane, where ISIS has an estimated 1,000 militants.

The US and its Arab allies have also been stepping up air strikes against ISIS in Syria.

On Thursday an AFP photographer across the frontier captured on film the moment another one apparently hit an ISIS position atop a hill west of Kobane.

A huge fireball dwarfed the few militiamen who had seen been manning the post under the jihadist group's black flag before all but one of them disappeared.

The aerial campaign has killed 553 people since its launch, the Syrian Observatory for Human Rights said, including 464 IS fighters and 57 militants from the Al-Qaeda affiliated Al-Nusra Front.

Thirty-two civilians have also been killed, including six children and five women, said the Britain-based Observatory, which relies on a wide network of sources inside Syria. The "vast majority" of militants killed were foreigners.