The US-led coalition's aircraft have destroyed the main water pipeline supplying the northern Syrian city of Raqqa, the de facto capital of the Islamic State (IS) group, according to activists.

Two bridges linking the city with the southern countryside were also destroyed amid fighting between the extremist group and US-backed fighters.

The coalition has been targeting IS in the area for more than two years and US-backed Kurdish-led fighters have been on the offensive in nearby areas, mostly north of Raqqa.

The city is the de facto capital of IS's self-declared caliphate and has been held by the extremist group since January 2014.

The attacks on Raqqa are similar to those conducted in Iraq late last year when Iraqi forces began their offensive to capture the city of Mosul from the extremists.

Destroying bridges makes it difficult for IS to move fighters but many complain that the adverse effects are worse for civilians.

Syrian state news agency Sana and an IS-affiliated website said early on Friday that the city's main water pipeline was damaged in an airstrike, cutting off the water supply to Raqqa.

The IS-affiliated website also reported the only two bridges on the Euphrates river on the southern edge of the city have been destroyed.

Raqqa is Being Slaughtered Silently, a local media collective, criticised the destruction of the two bridges, saying they are used by civilians and adding that such acts are "absolutely unacceptable".

The group said it appears that the US-led coalition is trying to "besiege the civilians of Raqqa by demolishing bridges and destroying the infrastructure".

The Britain-based Syrian Observatory for Human Rights, which relies on a network of on-the-ground activists to track Syria's war, reported clashes between IS fighters and the US-backed Kurdish-led Syria Democratic Forces (SDF) north of the city.

It said the US-led coalition has been providing air cover to SDF fighters during Friday's fighting.

There was no immediate word from the US-led coalition about targeting the water pipeline.

The attack comes at a time when Iraqi forces are on the offensive against IS in Mosul, where the US-led coalition has destroyed much of the bridges on the Tigris river.