Powerful Kurdish-led forces in northern Syria on Tuesday announced a push towards the Islamic State extremist group's de facto Syrian capital of al-Raqqa.

The Democratic Forces of Syria (DFS) intend to "liberate" the countryside north of the city, the Hawar News Agency, which is close to Syrian Kurdish forces, reported.

Kurdish official Idriss Nassan, based in the nearby city of Kobane, said the first stage of the campaign will not aim at entering al-Raqqa itself.

The Kurds have pushed the militants out of a swathe of northern Syria since the beginning of last year with intensive US air support, but analysts have questioned whether they would be well placed to capture al-Raqqa which has a mainly Arab population.

The Syrian Observatory for Human Rights said that DFS fighters, backed by US-led airstrikes, engaged Islamic State militants in heavy fighting north-west of al-Raqqa early Tuesday.

The announcement comes days after an unusual visit to northern Syria by the head of US armed forces in the Middle East, General Joseph Votel, who met DFS leaders.

The Observatory said the DFS offensive came after leaders received unspecified "promises" from Votel and US officials.

The Kurdish-led forces are positioned about 40 kilometres north of al-Raqqa, which together with Mosul in northern Iraq is thought to be one of the nerve centres of the extremist group's self-declared caliphate.

Kurdish advances on both sides of the border have cut deep into Islamic State's territory but the jihadists are thought to be well entrenched in the two cities.

The Democratic Forces of Syria is a coalition proclaimed by the Kurdish People's Protection Units (YPG) and smaller allied Arab groups.

In April, US President Barack Obama announced that he had approved the deployment of an additional 250 troops to Syria to support local forces in the fight against Islamic State.

An initial contingent of about 50 US special forces were sent to Syria late last year.

The US says the new troops will also aim at bolstering the capacity of the Arab components in the DFS.

US support for the Kurdish-led forces has angered the Syrian opposition, which accuses the Kurdish leadership of collaborating with the regime of President Bashar al-Assad - charges it denies.

Meanwhile, the Observatory reported that the death toll from a series of coordinated blasts targeting usually peaceful cities in Syria's regime-held coastal areas the day before has risen to 161. 

The Britain-based monitoring group said that 113 were killed in Jableh and 48 in Tartus. Supporters of Islamic State circulated claims of responsibility in its name on social media.

Four car bombs and five suicide attackers wearing explosive belts hit bus and taxi stations, a hospital, an electricity company office and groups of people in the two cities, according to the Observatory.

The attacks are considered the deadliest in years along the coastal region, which is firmly under government control and hosts hundreds of thousands of people fleeing conflict in Syria's inland provinces.

Syria's civil war began in 2011 with peaceful protests, but after a brutal government crackdown it spiralled into a four-way conflict between the al-Assad regime, mostly Islamist rebels including al-Qaeda jihadists, Islamic State, and the Kurds.

The conflict has cost a quarter of a million lives and displaced half the country's population, according to UN estimates.

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