TURKISH forces have captured a key Syrian border town under heavy bombardment, the military and a Syrian war monitor have said.

Turkish troops entered central Ras al-Ayn according to the Defence Ministry, marking the most significant gain since the invasion began Wednesday.

The ministry tweeted: “Ras al-Ayn’s residential centre 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 that it is Saturday and they are still there.

The fighting was ongoing as the Kurdish troops sought to reverse the Turkish advance into Ras Al-Ayn, the Syrian Observatory for Human Rights said.

The push deeper into northern Syria by Turkish troops came days after US President Donald Trump cleared the way for Turkey’s air and ground invasion, pulling back American forces from the area and saying he wanted to stop getting involved with “endless wars”.

President Trump’s decision drew swift criticism that he was endangering regional stability and risking the lives of Syrian Kurdish allies who brought down the Daesh group in Syria.

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

Earlier in the day, Turkish troops moved to seize control of key highways in north-eastern Syria, the Turkish military and the Observatory said.

Turkey’s state-run Anadolu news agency said that Turkey-backed Syrian opposition forces had taken control of the M-4 highway that connects the towns of Manbij and Qamishli.

The SDF said that Turkish troops and their Syrian allies reached the highway briefly before being pushed back again.

Turkish troops also cut the route linking the north-eastern city of Hassakeh with Aleppo, Syria’s largest city and once commercial centre, according to the Observatory.

Since Wednesday, Turkish troops and Syrian opposition fighters backed by Ankara have been advancing under the cover of airstrikes and artillery shelling, reaching the Manbij-Qamishli road about 19 miles south of the Turkish border. One such strike was the “mistaken” bombing of US special forces who came under Turkish artillery fire.

Navy Captain Brook DeWalt, a Pentagon spokesman, said: “Turkish artillery hit close to a US special operations unit near Kobani on Friday.

“The artillery shells hit several hundred metres from where the US special ops forces were located. There are no US injuries.”

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 UN estimated the number of displaced at 100,000 since Wednesday, saying that markets, schools and clinics also were closed.

Aid agencies have warned of a humanitarian crisis, with nearly 500,000 people at risk in north-eastern Syria.