Ceylandpinar, Turkey — Turkey's military said it captured a key Syrian border town under heavy bombardment Saturday as its offensive against Kurdish fighters pressed into its fourth day. Turkish troops entered central Ras al-Ayn, according to Turkey's Defense Ministry and a war monitor group, marking the most significant gain since the invasion began Wednesday.

The ministry tweeted: "Ras al-Ayn's residential center has been taken under control through the successful operations in the east of Euphrates" river.

Syrian Kurdish forces appeared to be holding out in some areas of the town. The Kurdish Syrian Democratic Forces released two videos said to be from inside Ras al-Ayn, showing fighters saying it is Saturday and they are still there.

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In addition to Turkey's assault, Syrian Kurdish forces have the ISIS threat to contend with. A Turkish shelled slammed into an ISIS prison compound, and moments later, ISIS prisoners were seen making a break for it, Charlie D'Agata reported. Kurdish forces had warned they would struggle to detain ISIS prisoners if Turkey attacked.

Turkey's push deeper into northern Syria — showing little sign of relenting despite mounting international pressure — comes days after President Trump cleared the way for Turkey's air and ground invasion, pulling back U.S. forces from the area and saying he wanted to stop getting involved with "endless wars." They decision drew swift bipartisan criticism that he was endangering regional stability and risking the lives of Syrian Kurdish allies who brought down ISIS in Syria.

The Kurdish-led Syrian Democratic Forces was the main U.S. ally in the fight against ISIS and lost 11,000 fighters in the nearly five-year battle against the extremists.

Earlier Saturday, Turkish troops moved to seize control of key highways in northeastern Syria, the Turkish military and the Observatory said. Turkish troops also cut the route linking the northeastern city of Hassakeh with Aleppo, Syria's largest city and once commercial center, according to the Observatory.

Turkey has said it aims to push back the Syrian Kurdish People's Protection Units, or YPG, which it considers terrorists for its links to a decades-long Kurdish insurgency within its own borders. The YPG is a main component of the SDF.

The U.N. estimated the number of displaced at 100,000 since Wednesday, saying that markets, schools and clinics also were closed. Observers say at least 30 civilians have been killed. Aid agencies have warned of a humanitarian crisis, with nearly a half-million people at risk in northeastern Syria.

Turkish President Recep Tayyip Erdogan said Friday Turkey won't stop until the Syrian Kurdish forces withdraw below a 32 kilometer (20 miles) deep line from the border.