U.S.-backed Syrian fighters advancing on the Islamic State from the eastern and western parts of the city of Raqqa have linked up for the first time since launching their offensive on the terrorist group’s de facto capital, officials said Friday.

Though the development marked a significant milestone in the battle for the Islamic State stronghold, a U.S. military spokesman cautioned that there is still tough fighting ahead before Raqqa is completely taken from the militants.

The Kurdish-led Syrian Democratic Forces, under the cover of U.S.-backed coalition airstrikes, launched a wide offensive to capture the city June 6 and have managed to take about half the city.

The linkup of the eastern and western fronts deprives the Islamic State from access to the Euphrates River and effectively leaves the remaining militants in Raqqa and thousands of civilians surrounded.

U.S. Army Col. Ryan Dillon said that there remains tough fighting ahead and that although the linkup of the Syrian opposition fighters’ on the southern edge of Raqqa is significant, it does not in fact cut the city in two.

Dillon said the development shows the steady progress SDF fighters are making against the Islamic State in the northern city, the militants’ self-declared capital.

“The fighting is ongoing from room to room and from house to house,” said Mustafa Bali, head of the SDF media center. Bali also confirmed that SDF fighters pushing from opposite sides of the city have met up.

Bali said by telephone from northern Syria that the key difficulty facing advancing SDF fighters is to avoid striking civilians used by the Islamic State as human shields.

The top U.S. envoy for the international coalition against the Islamic State, Brett McGurk, tweeted about the linkup of the two fronts, describing it as a “milestone” that is tightening the noose around the militant group’s fighters.

Also Friday, neighboring Turkey introduced new regulations at a border crossing with northwestern Syria, allowing only the transport of humanitarian aid, after an al-Qaeda-linked group took control of the Syrian post.

President Recep Tayyip Erdogan said after Friday prayers in Istanbul that the Cilvegozu crossing in southern Turkey would remain open for food, medicine and some supplies. Turkey’s Cilvegozu stands across from Bab al-Hawa in Syria’s Idlib province.

The al-Qaeda-linked militant group, the Levant Liberation Committee, captured the crossing after battles with the ultraconservative Syrian rebel Ahrar al-Sham group last month.