Story highlights Military official: Many ISIS targets in Kobani are too close to border for airstrikes

A witness says hundreds are trying to leave the Syrian city; "we'll get killed if we stay"

Many fighters on both sides have been killed in fighting, a witness says

Kurdish official: "We didn't choose this war, but we are obliged to fight"

Intense street fighting raged in the Syrian city of Kobani Monday as ISIS came closer to capturing a key area on the border with Turkey.

ISIS fighters planted their flag on a hill on the eastern side of Kobani, then punched through defenses to open up the route for more troops, one witness inside the city told CNN.

CNN crews on Monday also spotted what appeared to be the black flag of ISIS flying from a hilltop on the eastern side of the city. The flag was farther east into the city from one shown flying atop a building in video from Reuters and also seen by the CNN crews.

Many Kurdish forces defending the city were wounded and killed, and many ISIS fighters were also killed as clashes spread from street to street, the witness said.

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Map: Kobani (Ayn al-Arab)

The fall of the city would carry huge symbolic and strategic weight, giving ISIS sway over an uninterrupted swatch of land between the Turkish border and its self-declared capital in Raqqa, Syria, 100 kilometers (62 miles) away.

The Turkish military, which has bulked up its defenses along the border in recent days as the fighting has flared, blocked people fleeing the embattled city from crossing the border.

"We want to go across!" would-be refugees chanted as they pressed against a border fence.

One witness inside Kobani told CNN he'd been waiting to leave the city with hundreds of others since Sunday night.

"We'll get killed if we stay," he said.

As ISIS fighters attacked with tanks and heavy artillery, the city's defenders vowed to keep fighting.

"We are afraid of this. We are obliged to defend our home, our town," Kurdish Kobani official Idriss Nassan said. "We didn't choose this war, but we are obliged to fight."

U.S. and allies strike again

ISIS managed to close in on Kobani despite airstrikes by the United States and allied forces over the weekend and on Monday.

The most recent airstrikes took out two fighting positions near Kobani and two tanks near Raqqa, as well as two small ISIS units, two mortar positions and a building in Iraq, the U.S. military said Monday.

Over the weekend, allied airstrikes destroyed two ISIS tanks, a bulldozer and another ISIS vehicle, U.S. Central Command said. Two airstrikes hit a large ISIS unit and destroyed six firing positions, the U.S. military said.

A senior defense official said Monday to expect more airstrikes against ISIS targets in the Kobani area.

But that's easier said than done.

Another senior military official said many ISIS targets in Kobani are too close to the Turkish border or Kurdish forces to strike.

And the Pentagon, the official said, believes there's a media outcry about the situation in Kobani because reporters are there. Many other towns have fallen to ISIS without TV crews present, the official said.

No matter what role U.S. airstrikes play going forward, such attacks aren't enough, Nassan said.

"When I talk to people here in Kobani, they thank the international community, and the United States, they thank the countries who are striking the ISIS. But everyone believes it is not enough," he said.

The "international community cannot defeat ISIS by just hitting them from the sky. They have to help the people who are fighting -- the (Kurdish People's Protection Unit) YPG, the (rebel) Free Syrian Army who are here on the ground."

Meanwhile, about 270 kilometers (168 miles) to the east, in the northeastern Syrian city of Hasakah, ISIS suicide bombers drove trucks into checkpoints manned by Kurds, exploding them almost simultaneously, according to the Syrian Observatory for Human Rights, a London-based monitoring group.

Thirty Kurdish fighters died in the attack, the group said.

Helicopters used in Iraq strikes

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U.S. Army Apache gunships were used against ISIS targets in Iraq's Anbar province over the weekend, the senior defense official who spoke about the plans for more Kobani airstrikes told CNN.

Fixed-wing aircraft also were on the mission, but because Iraqi forces were close to ISIS forces, the Apaches were used because they fly low and can strike with precision, the official said.

The official described the area as "very much contested, with ISIS continuing to make gains."

The low altitude used by Apaches poses a risk, but with ISIS forces in such a populated area, their precision was needed, the official said.

U.S. officials don't see ISIS making a direct run for Baghdad, but concerns remain that ISIS elements are in the city and around the airport, the official said.

In other developments:

Total cost of U.S. airstrikes so far? More than $62 million, Pentagon says

U.S. military airstrikes against ISIS targets in Iraq and Syria have cost more than $62 million so far, according to data provided by the U.S. Defense Department.

The data, apparently sent out inadvertently Monday to the Pentagon's press contacts, lists the total number of airstrikes by U.S. Central Command in Iraq and Syria, details locations of targets and specifies the costs of munitions used.

Militant leader's fate unknown

The United States is still unsure if an airstrike killed the leader of the Khorasan Group. U.S. officials are still assessing the strike on the first night of airstrikes in Syria, and whether it killed Muhsin al-Fadhli, the senior defense official told CNN.

While he has not surfaced publicly or on social media, U.S. officials still have not seen sufficient corroborating intelligence, such as evidence of a funeral or martyrdom statements, the official said.

ISIS gets Pakistan Taliban encouragement

Pakistan Taliban spokesman Shahidullah Shahid issued a statement expressing solidarity with ISIS and other fighters in Syria.

"In these times when the enemy is standing against you we ask that you forget internal strife and rivalries and stand up against the nonbeliever army," Shahid said in the statement. "The Muslims of the world look to you with great expectation, and in this difficult time, we, your mujahidin brothers, are with you and will provide you with fighters and help," the statement said.

U.S. vice president apologizes

U.S. Vice President Joe Biden apologized to Turkey and the United Arab Emirates for saying Middle Eastern allies are partly to blame for the strengthening of ISIS.

At an appearance last week at Harvard University, he said the militant Islamist group had been inadvertently strengthened by actions allies took to help opposition groups fighting against Syrian President Bashar al-Assad.

"They poured hundreds of millions of dollars and thousands of tons of weapons into anyone who would fight against Assad. Except that the people who were being supplied were al-Nusra and al Qaeda and the extremist elements of jihadis coming from other parts of the world," Biden said.

On Turkey's alleged role, he said, "President (Recep Tayyip) Erdogan told me ... 'You were right. We let too many people (including foreign fighters) through.' Now they are trying to seal their border."

Erdogan vehemently denied ever saying such a thing.

Marine may be the campaign's first U.S. casualty

A Marine lost at sea after bailing out of an MV-22 Osprey when it appeared it might crash in the Persian Gulf is believed to be the first American military casualty in support of U.S. operations against ISIS in Iraq and Syria.

Cpl. Jordan L. Spears, 21, was declared dead after search and rescue efforts didn't find him.