Islamic State jihadists met dogged Kurdish resistance Sunday in the high-profile Syrian battleground town of Kobane but in Iraq they put forces under strong pressure, prompting US-led relief drops.

A roadside bomb killed the police chief in Iraq's Anbar province, between Baghdad and the Syrian border, where Pentagon officials have voiced concern about the vulnerability of government troops to a renewed jihadist offensive.

Farther north, around the key oil refinery town of Baiji, the Iraqi army and its Sunni Arab tribal allies came under fresh attack by IS, prompting a first resupply operation by coalition aircraft.

In Kobane, where IS is battling Kurdish fighters under the gaze of the international media just across the border with Turkey, the jihadists were taking heavy losses, the Syrian Observatory for Human Rights said.

It said IS was pouring in reinforcements from Syria, after its Friday capture of the Kurdish headquarters in Kobane failed to deliver a knockout blow.

"They are sending fighters without much combat experience," Observatory director Rami Abdel Rahman told AFP.

"They are attacking on multiple fronts but they keep being repulsed, then countering and being pushed back again."

IS has earned worldwide infamy for a spate of atrocities -- many of them videotaped and posted on the Internet -- since it seized swathes of Iraq and Syria in lightning offensives earlier this year.

But it has also gained prestige among Islamist extremists that has helped it recruit thousands of foreign fighters, a reputation that is now on the line in Kobane.

"It's a decisive battle for them," said Abdel Rahman.

"If they don't pull it off, it will damage their image among jihadists around the world."

In Cairo on Sunday, UN Secretary General Ban Ki-moon urged action to prevent a "massacre" in Kobane.

The UN has warned that hundreds of mainly elderly civilians in the centre and thousands more on the outskirts all risk massacre if the jihadists sever the sole escape route to the border.

The US military said Sunday that it and its Saudi and Emirati allies conducted four air strikes in Syria. All but one were in Kobane, where an IS fighting position was among targets destroyed.

Despite the air strikes, Pentagon officials have said there is a limit to what they can do without forces on the ground they can work with.

The Kurdish fighters in Kobane -- who have links with the outlawed Kurdistan Workers Party (PKK) over the border in Turkey, which is on Washington's terror blacklist -- say they have had no coordination with US commanders.

The United States said it was deeply concerned about the humanitarian risks of Kobane's fall but said it would keep to its overall strategy against IS, which prioritises the campaign in neighbouring Iraq.

"We know there's the threat of serious casualties -- that's why we're taking strikes in the Kobane area against (IS) targets," State Department deputy spokeswoman Marie Harf told AFP.

"What (IS) is doing in Kobane shows just how brutal these terrorists are. But the fight against (IS) is a much larger strategic effort than in any one town."

That strategy has seen Washington and its coalition partners carry out hundreds of air strikes in Iraq in support of its allies on the ground -- Kurdish forces in the north and embattled federal government troops farther south.

Coalition aircraft launched five strikes against IS in Iraq on Saturday and Sunday, the US military said.

Pro-government forces have come under particularly heavy pressure around the key oil refinery at Baiji, south of Iraq's second city of Mosul which the jihadists seized in June.

With the surrounding territory all in IS hands, the coalition airdropped ammunition, food and water to its besieged defenders on Friday and Saturday in the first such resupply operation to Iraqi forces, the Pentagon said.

US defence officials have expressed mounting concern about the tenuous position of Iraqi government troops in the face of the Sunni extremists of IS, particularly in the Sunni Arab heartland north and west of Baghdad.

Underlining that fragility, a roadside bomb on Sunday killed Anbar province police chief Major General Ahmed Saddag, officials said.

He had been en route to a battleground town near the provincial capital Ramadi, one of few remaining areas in Anbar still in government hands.

With federal troops on the back foot, Washington and its allies have relied heavily on Iraqi Kurdish forces in the fightback against the jihadists.

But they too have come under pressure.

Three suicide car bomb attacks targeting offices in a Kurdish-controlled town north of Baghdad killed at least 25 people Sunday, mostly Kurdish forces veterans volunteering to re-enlist, officials said.

IS claimed the attack via affiliated Twitter accounts, saying the bombers were from Germany, Saudi Arabia and Turkey.