The Syrian Democratic Forces (SDF) pushed closer to the Old City in the "Islamic State of Iraq and Syria" (ISIS) stronghold of al-Raqa on Monday (June 12th), the Syrian Observatory for Human Rights said.

The Arab-Kurd opposition alliance entered the northern city of al-Raqa for the first time almost a week ago, after battling for months to encircle it.

In the east of the city, they hold al-Meshleb neighbourhood, captured days after the operation inside the city began, and on Sunday they seized their first district in the west, al-Rumaniya.

On Monday, fighting was continuing on both fronts, with the SDF advancing quickly in the eastern neighbourhood of al-Senaa, which leads to the Old City of al-Raqa, the Observatory said.

"The SDF forces now control 70% of al-Senaa" next to al-Meshleb, according to Observatory director Rami Abdel Rahman.

"If they take al-Senaa it will be the most important advance in the battle for al-Raqa because it brings them to the centre of the city where the most important ISIS positions are," he said.

"When they have captured al-Senaa, the real battle will begin."

The fighting is expected to become more difficult as the SDF approaches the more densely populated centre of the city.

"This fight will be tough," SDF fighter Berkhdan Qamishli said Sunday.

"As we get close to the city centre, we will be fighting inside multi-storey buildings. Urban battles are tougher than fighting in villages, but we will fight until we control the whole city."

'Huge number of mines'

In the west of the city meanwhile, SDF forces were battling to enter Hatin, the neighbourhood next to al-Rumaniya, captured on Sunday.

The SDF reported "fierce clashes between fighters and the terrorists" on the two fronts and said 23 ISIS elements had been killed.

An SDF source said fighters had uncovered a series of tunnels dug by ISIS in al-Meshleb.

"We are moving carefully and cautiously to avoid the huge number of mines that ISIS has planted in the city," the source added.

Inside the west of the city, the approach was littered with mangled motorcycles and unexploded mortar rounds fired by ISIS, an AFP correspondent said, adding that the bodies of several alleged ISIS fighters could be seen on the streets.

Outside the city, fighting was continuing on Monday on the northern front, the Observatory said, where SDF progress has been slower.

ISIS has been using the "heavily fortified" Division 17 military base outside the northern city limits and an adjacent sugar factory to defend the northern approach into al-Raqa, the Observatory said.

Originally a Syrian army base, Division 17 was seized by ISIS in 2014 when it took control of swathes of the wider al-Raqa province.