A human rights monitoring group said more than 660 people have been killed in the month-long battle for the town of Kobani, along Syria's border with Turkey.
The British-based Syrian Observatory for Human Rights, with sources inside Syria, reported Thursday that 374 Islamic State militants have been killed fighting Kurdish forces in Kobani.
It said 258 Kurdish fighters, 10 others supporting the Kurds and 20 civilians have also been killed.
The observatory said it believes the overall death toll could be twice the number it gave, citing "extreme secrecy" about casualties from both the Islamic State and Kurdish forces.
The Pentagon said U.S.-led airstrikes around Kobani have killed "several hundred" Islamic State fighters, but that the town could still fall to the militants.
In Geneva, the United Nations High Commissioner for Human Rights, Zeid Raad al-Hussein, drew a comparison between the Islamic State advance in Syria and Iraq and the West African outbreak of Ebola, calling them "twin plagues" upon the world.
Zeid said they "both fomented quietly, neglected by a world that knew they existed but misread their terrible potential."
Meanwhile, the U.S. and Saudi Arabia intensified their strikes against the militants in Kobani this week, supporting Kurdish militias on the ground.
The U.S. military's Central Command said a total of 14 airstrikes were conducted Wednesday and Thursday near Kobani.
U.S.-led airstrikes began against Islamic State militants in Iraq in August and in Syria in September.
Central Command said, as of early Thursday, the anti-Islamic-State coalition has conducted a total of 524 airstrikes - 294 strikes in Iraq and 230 in Syria.
In addition to the U.S., countries taking part in airstrikes against Islamic State targets in Iraq include France, Belgium, Britain, Australia and the Netherlands; those taking part in Syria include Jordan, Bahrain, Saudi Arabia and the United Arab Emirates.
The Central Command is responsible for U.S. security interests in 20 nations that stretch from the Arabian Gulf region into Central Asia.
The Pentagon said that while the airstrikes, an operation now dubbed "Inherent Resolve," have helped the Kurdish militia defenders maintain their tenuous hold, the town could still fall to Islamic State fighters.
The United States also said Thursday it has held its first direct talks with a Syrian Kurdish political party with ties to Kurdish guerrillas whom the U.S. and Turkey call terrorists.
State Department spokeswoman Jen Psaki said a top U.S. official met with the Kurdish Democratic Union Party in Paris.
Psaki stressed that this is just one meeting and does not represent coordination in the fight against Islamic State.
The party is tied to the PKK - Kurdish guerrillas who have carried out numerous attacks inside Turkey while fighting for an independent Kurdistan in Turkey, Iran, Iraq and Syria.
Psaki said the U.S. is fully aware of the connection between the Kurds in Syria and the PKK, which Washington still regards as a terrorist group.
Flow of militant fighters
Pentagon spokesman Rear Admiral John Kirby said the Islamic State group has been sending fighters into Kobani in greater numbers and that presents more targets for coalition aircraft.
"They have continued to flow fighters to Kobani, meaning there are more targets in and around Kobani. So, one of the reasons you’re seeing so many strikes there is because there's more ISIL there. We believe, and it’s hard to give an exact number, that we have killed several hundred ISIL fighters in and around Kobani," said Kirby, using an acronym for the Islamic State group.
Kirby said the Kurdish defenders are fighting hard to keep the town out of IS hands, and that the air strikes have helped their defense.
"We do believe our air strikes have helped in that, that ISIL still threatens Kobani, but that they’re holding it. Right now, we believe it is still being defended and in their hands, the Kurdish militia. That doesn’t mean there aren’t going to be pockets of Kobani that ISIL controls or temporarily has possession of. It’s a fluid situation but, by and large, our assessment today is that the Kurdish militia still holds it," said Kirby.
But, he acknowledged, the town could still fall to Islamic State militants. That would be a setback.
President Barack Obama’s special envoy for the Global Coalition to Counter the Islamic State group, former Marine General John Allen, said the coalition airstrikes in both Iraq and Syria are aimed at slowing the tactical momentum of the Islamic State group.
"And that, in fact, has occurred in some areas. They still retain some tactical momentum in other areas. And that’s to be expected. In some areas, Amerli, Mosul Dam, Haditha, those airstrikes were very helpful," Allen said.
"We’re actually focusing, obviously, around Kobani, providing airstrikes to provide humanitarian assistance and relief there, obviously to give some time to the [anti-Islamic State] fighters to organize on the ground. But, in Anbar province [in western Iraq], our hope is to stop or halt that tactical initiative and momentum that they have there," he said.
Fighting on Thursday
Heavy and light weapons fire were audible from across the border in Turkey on Thursday afternoon, with one stray mortar hitting Turkish soil close to abandoned tents, a Reuters correspondent said.
Turkish security forces moved civilians and media away from hills overlooking Kobani as the fighting raged.
Six airstrikes hit eastern Kobani and there was fierce fighting between Kurdish and Islamist fighters overnight on Wednesday, but neither side made significant gains, according to the human rights group.
The Islamic State offensive on Kobani that began in September has driven tens of thousands of mostly Syrian Kurds to flee across the border into Turkey.
Allen said the airstrikes are giving Iraq time to train its refurbished security forces, as well as train and equip free Syrian fighters.
Pentagon spokesman Kirby expects a long fight against the Islamic State group and declined to say the U.S.-led coalition is losing ground to the militants.
He acknowledged the results, so far, have been mixed.
"There’s going to be setbacks. There’s going to be wins and there’s going to be losses. And we’re mindful of the complicated nature of this," Kirby said.
However, he pointed out the Iraqi defense of Baghdad has stiffened and the capital remains secure.
Iraq town could fall
Security forces are battling Islamic State fighters for control of a town 35 kilometers west of Baghdad: Amriyat al-Fallujah, in Anbar province.
Amin Saikal, director of the Center for Arab and Islamic Studies at the Australian National University, said despite Pentagon assurances, Baghdad remains threatened by Islamic State fighters.
"Let’s hope that the focus on Kobani has really not diverted the attention of the coalition forces from the fact that ISIL has made progress in its drive to move closer to Baghdad, and there have been reports that ISIL forces are getting nearer to the Iraqi capital and that could be quite ominous and some analysts have predicted they may even be able to knock on the doors of the Iraqi capital very soon," Saikal said.
Saikal said new Iraqi Prime Minister Haidar al-Abadi’s government is not fully functional, making it easier for outside forces like the Islamic State group to take advantage of what he called the “political vacuum” in the city.
VOA's Victor Beattie contributed to this report from Washington. Some material for this report came from Reuters.