Syrian Kurds and allied rebels advanced against the Islamic State group on Tuesday, capturing a strategic town a day after seizing a base from the jihadists near their Raqa bastion.

A spokesman for the Kurdish People's Protection Units (YPG) and a Britain-based monitor said anti-IS forces took Ain Issa after capturing the nearby Brigade 93 base overnight.

"Ain Issa has come under our full control, along with dozens of villages in the surrounding area," YPG spokesman Redur Khalil told AFP.

The Syrian Observatory for Human Rights, a Britain-based monitor, said IS had withdrawn from the town and YPG and rebel forces were checking for mines laid by the jihadists.

Ain Issa's fall comes after IS ceded control of the Brigade 93 base late Monday and the border town of Tal Abyad more than a week ago.

Ain Issa and Brigade 93 are around 55 kilometres (35 miles) north of Raqa, de facto capital of IS's self-declared Islamic "caliphate" in Syria and Iraq.

They both lie on a main highway between Kurdish-held territory in Aleppo province to the west and Hasakeh province to the east.

The same route links territory held by IS in Aleppo and Hasakeh provinces.

"It's also a defence line for Raqa," said Mutlu Civiroglu, a Kurdish affairs analyst.

"Considering that Raqa is a sort of capital of the 'caliphate', it creates a lot of pressure on IS."

- IS 'pushed back' -

The YPG-rebel advance has been backed by air power from the US-led coalition fighting IS, with the Observatory saying at least 26 jihadists were killed in international strikes in and around Ain Issa on Monday.

The monitor said on Tuesday that at least 2,896 people -- mostly IS jihadists -- had been killed in coalition strikes in Syria since the air campaign began on September 23, 2014.

The toll included 2,628 IS members, mostly foreign fighters, as well as 105 fighters from IS's rival jihadist group the Al-Nusra Front, and one Islamist fighter.

According to the Observatory, coalition strikes have also killed 162 civilians, 52 of them children, in Syria.

The Pentagon has acknowledged just two civilian deaths in Syria in the international campaign against IS.

Observatory head Rami Abdel Rahman said IS's defence lines had been "pushed back to the outskirts of Raqa city".

The capture of Tal Abyad on June 16 cut off a key conduit for IS, which had used the border town to bring in fighters and weapons from Turkey and export black market oil.

Kurdish forces have been chipping away at IS territory in the northern Raqa province for months, after successfully repelling a fierce jihadist attack on the border town of Kobane in January.

The YPG has emerged as "arguably the most effective fighting force against IS in Syria", analyst Sirwan Kajjo said after Tal Abyad's capture.

- IS destroys mausoleums -

Khalil declined to comment on where the anti-IS fighters would focus their attention next, but suggested an operation against Raqa was unlikely soon.

"Raqa is much further away, and well-defended, it would require significant forces and weapons," he said.

Civiroglu also said any offensive against Raqa would require lengthy planning and additional weapons for the YPG and its allies, who would opt to consolidate their hold on Tal Abyad and surrounding areas.

On Tuesday, IS released a gruesome video showing the murders of 16 men it accused of being "spies".

It showed the jihadists drowning the men, decapitating them with explosives, and piling them into a car and firing a rocket-propelled grenade at it.

The video came after IS published photos of the destruction of two ancient religious mausoleums in Syria's historic city of Palmyra.

The jihadists consider tombstones and mausoleums to be a violation of their strict interpretation of Islamic law.

Syria's four-year war has left more than 230,000 people dead and forced millions to flee their homes.

Both government and opposition forces have been criticised for indiscriminate attacks that have killed civilians.

On Tuesday, UN investigators denounced the "unspeakable suffering" of civilians caught under barrel bomb attacks or in besieged towns.

"Civilians are the main victims of an ever-accelerating cycle of violence," Paulo Pinheiro, who heads a commission of inquiry on rights in Syria, told the UN Human Rights Council.

He said warring parties had taken "seemingly deliberate decision to put civilians in harm's way".

IS spokesman Abu Mohammed al-Adnani, in an Internet audio message Tuesday marking the Muslim holy month of Ramadan, urged people who had fled violence in Iraq's Anbar province to return home.