US National Security Adviser Susan Rice announced in a television interview that Turkey had agreed to let the US and its allies use its bases to conduct activities inside Syria and Iraq.

A US-led military coalition has been bombing fighters from the self-styled "Islamic State" in Iraq and Syria and the discussion continues within the US military whether to join ground combat against the militant extremists in Iraq and Syria.

US Defense Secretary Chuck Hagel's spokesman said that Hagel had thanked his Turkish counterpart Ismet Yilmaz for Turkey's "willingness to contribute to coalition efforts, to include hosting and conducting training for Syrian opposition members."

The Associated Press news agency reported that Turkey authorized the training of up to 4,000 opposition fighters on its soil, once they had been screened by Turkish intelligence agencies.

Pressure on, movement from Ankara

US troops have long operated out of Incirlik Air Base, with roughly 1,500 air force personnel stationed there. The US airstrikes against "IS" are currently reportedly flying out of air bases in the United Arab Emirates, Kuwait and Qatar.

NATO member Turkey had so far refused to get involved in the fighting, calling instead for a joint strategy against the militants. At a donors' conference for the Gaza Strip on Sunday evening, US Secretary of State John Kerry stressed the seriousness of the situation in the border town of Kobani.

"We are all very concerned about the reports of gains in Kobani and we're closely monitoring the situation. In fact, we're not just monitoring it, we've been deeply engaged with strikes in the last days and today there were more strikes," Kerry said in Cairo.

Meanwhile Kurdish fighters are continuing their defense of the border town of Kobani, from where 200,000 refugees have fled and crossed into Turkey in recent weeks.

The United Nations has warned of mass casualties if the border town, officially called Ayn al-Arab, falls.

msh/rg (AFP, AP, Reuters)