A UN official warned of pending "humanitarian tragedies" and pleaded desperately with the world to intervene on behalf of Kurds trapped in a Syrian city near the Turkish border, as Islamic State fighters stood on the brink of taking it.

Kurds from villages throughout northern Syria have fled to Kobani for a final stand as the terrorist group has marauded across huge swaths of land, leaving a trail of death and destruction. With the city under siege for three weeks, the black-clad fighters have begun to raise their flag over neighborhoods and UN Special Envoy for Syria Staffan de Mistura said the city was about to fall.

"The world has seen with its own eyes the images of what happens when a city in Syria or in Iraq is overtaken by the terrorist group called ISIS or Da'esh: massacres, humanitarian tragedies, rapes, horrific violence," De Mistura said. "The international community cannot sustain another city falling under ISIS.

"The world, all of us, will regret deeply if ISIS is able to take over a city which has defended itself with courage but is close to not being able to do so," De Mistura added. "We need to act now."

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With the fighting taking place with view of the Turkish border, international pressure increased on Ankara to get involved militarily. Turkey has already taken in an estimated 200,000 refugees, and Turkish President Recep Tayyip Erdogan said the coalition air campaign launched last month would not be enough to halt the Islamic State group's advance.

"Kobani is about to fall," Erdogan told Syrian refugees in the Turkish border town of Gaziantep, according to The Associated Press.

Erdogan, whose troops are massed near the border but have so far not taken an offensive posture, called for greater cooperation with the Syrian opposition, which is fighting both the extremists and forces loyal to Syrian President Bashar Assad.

"We asked for three things: one, for a no-fly zone to be created; two, for a secure zone parallel to the region to be declared; and for the moderate opposition in Syria and Iraq to be trained and equipped."

The capture of Kobani would give ISIS control of a large swath of land bordering Turkey and eliminate a vital pocket of Kurdish resistance. It would also provide a link between the group's territory near the ancient Syrian city of Aleppo and its largest operations base at Raqqa in northeastern Syria.

The Associated Press reported that warplanes believed to be part of the U.S.-led coalition against Islamic State, struck militant positions Tuesday. Journalists on the Turkish side of the border heard the sound of warplanes before two large plumes of smoke billowed just west of Kobani. A Fox News crew on the Turkish side of the border reported only one U.S. airstrike in the previous five days.

Fighting continued into Tuesday morning on the outskirts of the town. One coordinator with the Kurdish defenders told The New York Times that their defenses benefited from the new round of airstrikes, but they were still outmatched by the more heavily armed militants.

On Tuesday morning, the AP reported that occasional gunfire could be heard in Kobani, also known by the Arabic name Ayn Arab. A flag of the Kurdish force known as the People's Protection Units, or YPG, was seen flying over a hill in the center of town.

The Wall Street Journal reported that ISIS fighters had entered the eastern outskirts of the city on Monday after capturing more than 300 surrounding Syrian Kurdish villages in the previous three weeks. The paper also reported that the militants raised their black flag in two separate places, one on top of a civilian apartment building and another on a hilltop near a checkpoint at the city’s eastern entrance. The flag at the checkpoint could be seen by reporters watching from across the border in Turkey.

The Britain-based Syrian Observatory for Human Rights said the Kurds forced the jihadists to withdraw from the eastern part of the town in heavy clashes after midnight Tuesday, adding that five loud explosions were heard in the town as warplanes soared overhead. However, a local Kurdish militia commander estimated to The Journal that ISIS fighters were still a mile from the city center. On Tuesday, Reuters reported that ISIS had also taken over several buildings in the southwest of the city.

Before the recent fighting began, the city had been a focal point for refugees fleeing Syria's three-years-long civil war. Between 160,000 and 180,000 people are believed to have fled into Turkey since the ISIS advance began. A Kurdish politician told Reuters that more than 2,000 Syrian Kurds, including women and children had been evacuated from the town in the midst of the fighting Monday.

State Department officials told the Journal that U.S. officials will travel to Turkey later this week to discuss the status of the international coalition. Retired Marine Gen. John Allen, the White House's special envoy in the fight against ISIS, is among those traveling to Turkey.

Despite U.S. pressure to become a full-fledged member of the coalition, and despite the Turkish parliament passing a law giving the government authority to conduct operations against ISIS in Syria or Iraq, Ankara has largely stayed on the sidelines. Fox News reported Monday that twenty Turkish tanks have been stationed on a hillside overlooking Kobani, ready to strike the city on short notice. However, Turkish authorities have mostly been preoccupied with attempting to control the flow of Kurdish refugees across the border and deal with their protests at the government's inaction.

Also Tuesday, Turkish media reported that police in Istanbul and at least six other Turkish cities clashed with hundreds of demonstrators. The private Dogan news agency clashes broke out in several Istanbul neighborhoods overnight, as protesters set up barricades, hurled stones, fireworks and firebombs at police and set a bus on fire. One police officer was injured.

Police used tear gas and water cannon to disperse similar protests in the mostly Kurdish-populated cities of Diyarbakir, Batman, Van, Sirnak, Sanliurfa and Hakkari.

Fox News' Greg Palkot and The Associated Press contributed to this report.