US-backed Syrian fighters have reached a major dam held by the Isis group in the north of Syria, according to officials.

Cihan Sheikh Ehmed, spokeswoman for the Syrian Democratic Forces (SDF), said the fighting was continuing at the entrance of the Tishrin Dam on the Euphrates river, adding that there are casualties among Isis fighters.

The Britain-based Syrian Observatory for Human Rights said SDF fighters were moving slowly towards the dam because of mines and explosives planted by Isis.

The push comes three days after US aircraft ferried Syrian Kurdish fighters and allies behind Isis lines to spearhead a major ground assault on the Isis-held town of Tabqa, where the dam is located, west of Raqqa.

US-backed fighters are also pushing to capture a major village east of Raqqa, the de facto capital of Isis.

The Kurdish-led SDF has been on the offensive since November, aiming to surround the city.

The Syrian Observatory for Human Rights said SDF fighters have been marching under air strikes and artillery cover from the US-led coalition, and are inside the village of Karama.

Mohammed Khedhr, of Sound and Picture Organisation, which documents Isis violations, confirmed that battles are taking place inside Karama, about 10 miles east of Raqqa.

France’s defence minister said the battle for Raqqa itself will start in the coming days.

Jean-Yves Le Drian said Raqqa is a “major objective” for the US-led coalition, adding: “Today we can say that Raqqa is encircled and that the battle will begin in the coming days. It will be a very hard battle but it will be an essential battle.”

Syria’s UN ambassador said hundreds of US personnel are “invading my country” and insisted any effort to liberate the city should be done in coordination with the Damascus government.

Bashar Jaafari also insisted that “American warplanes” had bombed a school in the village of Mansoura, west of Raqqa, a day earlier and were responsible for the deaths of 237 civilians among 500 people fleeing the city.

The US has deployed more than 700 advisers, Marines and Rangers to Syria to support fighters battling Isis.

Mr Jaafari was speaking at the start of a new round of UN-mediated peace talks.

UN envoy Staffan de Mistura hosted Mr Jaafari at UN offices in Geneva for the talks set to take place over at least several days around issues of governance, elections, a new constitution and the fight against terrorism.