Islamic State (Isis) fighters have moved deeper into the Syrian town of Kobani on the Turkish border and may well capture it, said the deputy US national security adviser.

Tony Blinken said the militants controlled 40% of the town, echoing figures from the UK-based Syrian Observatory for Human Rights, which said Isis had taken almost complete control of an area where the local Kurdish administration was based.

“I don’t know what’s going to happen because again in the absence of any ground force there, it is going to be difficult just through air power to prevent [Isis] from potentially taking over the town,” Blinken said.

By mid-morning on Friday, occasional gunfire and explosions that appeared to be rocket-propelled grenades and mortar shells could be heard from across the border in Turkey, and plumes of smoke were seen rising in the distance.

The US military said it made seven air strikes around Kobani on Thursday and Friday. Two Isis training facilities were hit south-east of Kobani, four strikes hit south of Kobani and one air strike to the north destroyed Isis vehicles, a statement said.

Blinken said there would be other similar situations to Kobani where US actions may or may not be effective as it leads a coalition against Isis. “There are other Kobanis in Iraq, there are other Kobanis in Syria on a daily basis,” he said.

The new UN envoy to Syria warned that at least 500 civilians trapped in Kobani could be massacred if the town falls.

Staffan de Mistura said in Geneva that a UN analysis showed only a small portion of Kobani remained open for people to enter or flee the town.

De Mistura said there were about 500 – 700 elderly people and other civilians still trapped in Kobani, while 10,000–13,000 were stuck in an area nearby, close to the Syrian-Turkish border.

A Kurdish official in Kobani, Idriss Nassan, said on Friday that Isis had shelled a border crossing in an attempt to take it and cut off the embattled town.

Isis pushed into Kobani this week for the first time since launching its offensive in the area in mid-September. The onslaught has forced more than 200,000 people to flee across the border into Turkey and activists say the fighting has killed more than 500 people.

“Daesh [Isis] is doing all it can to take the border crossing point through the farmlands east of the city,” Nassan said. “They think there might be help [for the Kurdish militia] coming through the crossing so they want to control the border.”