The Syrian army backed by Russian air strikes has opened a major new front against Islamic State, the third big assault on the self-proclaimed caliphate this week after Iraqi forces attempted to storm a city and a Syrian militia advanced with US support.

The week's three big offensives are some of the most aggressive campaigns against Islamic State since it declared its aim to rule over all Muslims from parts of Iraq and Syria two years ago. They signal apparent new resolve by the group's disparate foes on a range of fronts.

Heavy Russian air strikes hit Islamic State-held territory in eastern areas of Syria's Hama province, near the boundary of Raqqa province on Friday. Raqqa city, further east, is Islamic State's de facto capital in Syria and, along with Mosul in Iraq, the ultimate goal of those seeking to destroy the group's rule.

The Syrian army had advanced some 13 miles and was now near the edge of the provincial boundary, according to the Syrian Observatory for Human Rights, a group that monitors the war.

Separately, US-backed militias have been pressing a multi-pronged attack against Islamic State in other parts of Raqqa province and neighboring Aleppo province.

This week, they began a push toward the city of Manbij near the Turkish border, aiming to seize the last 50-mile stretch of Turkish-Syrian frontier under Islamic State control and cut the group's main external link for manpower and supplies.

The US military said on Friday its allies were advancing against heavy resistance from Islamic State. If successful, the Manbij campaign would free 40,000 civilians from Islamic State control.

The YPG and its Arab allies, who formed the Syria Democratic Forces (SDF) last year, have proven to be the first force in Syria allied to the United States that has been effective in fighting against Islamic State.

The SDF has taken 28 villages from Islamic State in its push toward Manbij, the Syrian Observatory said, and had freed more than a dozen women from the Yazidi minority who were taken by Islamic State fighters from Sinjar in Iraq.

US President Barack Obama has authorized several hundred special forces troops to operate in Syria, some of whom are deployed as advisers in the latest advance.

The Kurdish fighters' progress has been limited in the past by Turkey, which considers them enemies. But Ankara has signaled its tacit support for the latest advance, saying it understands most fighters involved will be Arabs, not Kurds.

"RACE FOR RAQQA"

The Syrian army's new offensive was described in a pro-Damascus Lebanese newspaper as part of "the race for Raqqa" - with the government and its Russian allies trying to advance on Islamic State's de facto Syrian capital before it falls to the fighters allied to the Americans.

A Syrian military source played this down. Reports the offensive targeted Raqqa were only "expectations", he said, and both Raqqa and Deir al-Zor, another Islamic State-held city in eastern Syria, were possible targets.

Syrian government warplanes killed at least 15 people in air raids on the town of al-Boulil in the eastern province of Deir al-Zor on Friday, the Syrian Observatory said, saying four women and a child were among those killed.

Whatever its ultimate target, the offensive appears to be the biggest Damascus has mounted against Islamic State since it recaptured the city of Palmyra with Russian support earlier this year. In the past, the United States has accused Assad and his Russian backers of ignoring Islamic State to take on other foes.