Water to the Islamic State group's Syrian bastion of Raqa has been cut after suspected coalition raids on the city's main pipeline, a monitor and activists told AFP on Friday. 

"After coalition air strikes, the main water line was ruptured and water was cut to all of Raqa city," said Hamoud al-Mousa of the Raqa is Being Slaughtered Silently activist collective. 

His group said the overnight strikes hit several bridges in the north of Raqa, including one known as the Old Bridge.

Mousa said a pipeline running along the Old Bridge into Raqa -- the only way residents could get water -- had been destroyed. 

"People were heading to the river at dawn today to get water," Mousa told AFP. 

Raqa, 90 kilometres (less than 60 miles) south of the Turkish border, is IS's de facto capital in Syria and remains home to more than 300,000 people. 

The city and surrounding province have been battered by air strikes carried out by the US-led coalition fighting IS and by Russia, a key ally of Syria's government. 

The coalition raids are backing an alliance of Arab and Kurdish fighters who launched a major offensive against Raqa in November. 

The Syrian Observatory for Human Rights confirmed water to Raqa had been cut after "probable" coalition raids. 

"Air strikes on the city destroyed a huge water pipe. It's almost certain they were coalition raids," said Observatory head Rami Abdel Rahman.

The Observatory, which relies on a wide network of sources inside Syria for its information, says it determines whose planes carry out raids according to type, location, flight patterns and munitions used.