BEIRUT (Reuters) - U.S.-backed forces seized control of the last route into Islamic State-held city of Manbij in northern Syria on Friday, completing their encirclement of the main target in a major advance against the militants, a monitoring group said.

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The Syria Democratic Forces, supported by U.S.-led air strikes and American special forces, launched and advance last week to seize Islamic State’s last territory on the Syria-Turkey border and cut the self-declared caliphate off from the world.

Other enemies of Islamic State, including the governments of Syria and Iraq, also launched major offensives on other fronts, in what amounts to the most sustained pressure on the militants since they proclaimed their caliphate in 2014.

Officials of the SDF, a U.S.-backed group formed last year to unite powerful Kurdish militia with Arab anti-Islamic State fighters, could not immediately be reached.

The SDF had by Thursday advanced to within firing distance of the last main highway into Manbij, Islamic State’s main bastion in the border area west of the Euphrates. The Syrian Observatory for Human Rights monitoring group said the SDF had effectively taken control of the final road to the city early on Friday.

“There’s no road left... they’re all cut,” the Observatory’s director, Rami Abdulrahman, said.

The offensive near the border is the most ambitious advance yet in Syria by a group allied to Washington, which has previously struggled to develop capable allies on the ground amid Syria’s five-year multi-sided civil war.

SDF forces have also advanced in the neighboring province of Raqqa, and Syrian government forces and their allies, backed by Russia, opened a separate front against Islamic State in Raqqa province last week.

The Iraqi government has launched its own assault on the Islamic State bastion Falluja, an hour’s drive from Baghdad, at the opposite end of Islamic State territory.

The SDF has said this week that it was holding back from an immediate assault to enter Manbij out of concern for civilians.

The Observatory said nearly 160 Islamic State fighters had died in battles with the SDF around Manbij and more than 20 SDF fighters had been killed.