REUTERS - Islamic State militants advanced on Iraq's Mount Sinjar on Tuesday, tightening a siege of thousands of stranded Yazidis, who called on the United States and its allies to act to avert more bloodshed.

The attack is the latest threat to minority Yazidis, thousands of whom have been shot, buried alive or sold into slavery by Islamic State militants, who regard them as devil-worshippers.

The IS militants originally attacked the area around Sinjar, in northwestern Iraq, in August. A renewed assault began at dawn on Monday, when militants driving Humvees and civilian vehicles attacked several Yazidi residential compounds, forcing the Yazidi to retreat up the mountain.

"We are outnumbered and outgunned. We don't know how long we can hold them off," said Ali Qasem, a Yazidi volunteer on the mountain.

Qasem said most families had already fled by the time IS arrived, but some could not leave and remained trapped in residential complexes to the east of the mountain.

U.S. President Barack Obama authorized air strikes in Iraq in August, citing the duty to prevent an impending genocide of Yazidis at the hands of IS militants after they overran a vast swath of northern Iraq.

The air strikes helped Kurdish forces turn the tide against IS in the north and relieved some of the pressure on Sinjar so that a corridor could be opened to evacuate thousands of Yazidis from the mountain.

However, the mountain is still under threat, and the air strikes have not prevented IS from gaining ground elsewhere in Iraq as well as neighbouring Syria, where they have been attacking the predominantly Kurdish town of Kobani, also known as Ayn al-Arab.

A Yazidi parliamentarian questioned why U.S. planes were striking IS positions in Kobani but not Sinjar. He said the militants sought to control the mountain to gain a strategic refuge near the border with Syria.

"Unfortunately, coalition planes are in the sky and can see the tanks, but they are not striking them," said Yazidi parliamentarian Mahama Khalil, also on the mountain. "Why do they defend Kobani and not Sinjar?"

Monday's attack was one of several along the front line between IS and Kurdish forces, which runs more than 1,000 kilometres from the Syrian border to Iraq's eastern Diyala province, near the frontier with Iran.

Militants disguised in Kurdish clothes tried to overrun the town of Qara Tapa in Diyala on Monday and later drove a tanker rigged with explosives into Kurdish lines in the Wana district, killing as many as 15 peshmerga, according to two officials.

Wana is around 40 km northwest of Mosul, near Iraq's largest dam, which was seized by IS in August before the U.S. air strikes drove the militants back and Kurdish peshmerga retook it.

A senior civil servant in the office of the Kurdistan region's president said recent IS attacks in the north aimed to divert the Kurds' attention to create an opening for another offensive against the oil-rich city of Kirkuk and Khanaqin town in Diyala.

"They have strategic interests there, too, but their grand strategy is Kirkuk and other Kurdish areas in Diyala to encircle Baghdad," Ari Mamshae said.

In a statement late on Monday, IS said it had carried out an attack on the "secularist Yazidi militias" and destroyed one of their shrines.

"The advances are continuous and the armies that liberated Nineveh and Ayn al-Arab are taking another step towards the Mosul dam, to which the mujahideen are very close."