Beirut (dpa) - US-backed Syrian forces on Thursday entered Minbij, a strategic town controlled by Islamic State near the Turkish border, a monitoring group reported.

The Kurdish-led Democratic Forces of Syria (DFS), backed by US airstrikes, stormed the town from the south-west, according to the Syrian Observatory for Human Rights.

Fierce clashes were under way between the two sides in the western part of the northern Syrian town, the watchdog said, without reporting casualties.

"The battle for controlling Minbij will be long for the DFS because Daesh has for sure planted explosives in the town," Observatory chief Rami Abdel-Rahman told dpa, using an Arabic acronym for Islamic State.

He said that DFS forces were advancing into the town with "caution," fearing militants' typical traps of car bombs and suicide attackers.

The violence forced an exodus of locals from Minbij to safer areas, according to the Britain-based watchdog group, which relies on a network of activists inside Syria.

The Observatory estimated that around 8,000 people have fled from Minbij in the past few days when fighting escalated on its outskirts.

Minbij is strategically important for its location along a supply route from the Turkish border to al-Raqqa, Islamic State's de facto capital in Syria.

On May 31, the DFS, an alliance of Kurds and Arabs, started an offensive to seize Minbij from the al-Qaeda splinter group.

Syrian rebels lost Minbij to Islamic State forces in early 2014 after they had seized the city from Syrian President Bashar al-Assad's forces in 2012.

Meanwhile, a top UN official warned that more than 5 million Syrians now live in hard-to-reach areas, an increase of 900,000 from a previous estimate.

The increase is due to heightened insecurity and constrained humanitarian access in Aleppo, Raqqa and Hasakeh governorates, UN humanitarian affairs chief Stephen O'Brien told the UN Security Council in New York.

He said that humanitarian access has somewhat improved this year with the UN having reached more than 844,000 people living in besieged and hard-to-reach areas. But the progress was a "trickle compared to the level of protection concerns, needs and suffering" in those regions O'Brien said.

"It is therefore vital that the stuttering momentum sustained on humanitarian access over the past few months continues and improves, and is significantly expanded into the second half of the year," he said.

The UN has requested humanitarian access from the Syrian government to 35 besieged and hard-to-reach areas for the month of July, O'Brien said.

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