United Nations Secretary-General Ban Ki-moon urged the international community Sunday to prevent a “massacre” in the Syrian city of Kobani, where Kurdish forces and Syrian rebels fighting with the Free Syrian Army are battling the Islamic State group. The Sunni militants sent reinforcements to step up its offensive around Kobani Sunday, Agence-France Presse reported.

“Thousands of lives are at stake. I once again call on all the parties to stand up to prevent a massacre of civilians in Kobani,” Ban said. Sunday marked the first time the U.N. leader spoke publicly about the fighting in Kobani, which has become the main focus of the offensive in Syria by the militant group formerly known as ISIS.

If the Islamic State group were to capture Kobani, it would control a piece of land in northern Syria in the vicinity of one of the major oil transit routes into Turkey. Kurdish militias have been fending off ISIS with the help of U.S.-led coalition airstrikes, but have called on the international community for more support.

Despite the Kurdish pleas for help, the Turkish military has not intervened. Its tanks sit on the border and watch the fighting from afar. President Recep Tayyip Erdogan has said he would not send Turkish ground troops in without the establishment of both a buffer zone and a no-fly zone.

However, Kurdish militias in Kobani have received support from the Free Syrian Army. More than 50 rebels from Raqqa, now an Islamic State stronghold, have moved into the city to help keep it from falling. A small contingent of female Kurdish fighters are also helping the Kurdish militias by launching an offensive to take back the West Gate of Kobani.

The Turkish government agreed Friday to train 2,000 Free Syrian Army rebels, but did not agree to send troops into Kobani. The Syrian rebels will be trained in groups of 400. Turkey’s rigorous National Intelligence Program will select and screen them before they enter Turkey.

Turkey has been accused in recent weeks of letting ISIS fighters through the border. However, Turkish officials told the International Business Times that the government has implemented a robust plan to catch Islamic State fighters and their resources at the main border crossings. Meanwhile, that plan is hurting Kurdish fighters in Kobani, who have said Turkey is not allowing Kurdish supporters to crossing into the city to fight.

Activists said Sunday that, despite its comparative lack of fighters and heavy weaponry, the Kurds have stopped ISIS from reaching the city center, the Associated Press said.