Militants of the Islamic State group were closing in Monday on a Kurdish area of Syria on the border with Turkey — an advance unhindered so far by U.S.-led coalition airstrikes, including one that struck a grain silo, killing two civilians, according to activists.

Islamic State fighters pounded the city of Kobani with mortars and artillery shells, advancing within three miles of the Kurdish frontier city, according to the Britain-based Syrian Observatory for Human Rights and Nawaf Khalil, a Kurdish official.

The Islamic extremists intensified their shelling of the border region following U.S.-led strikes Saturday. The aerial assault appeared to have done little to thwart the militants, Kurdish officials and activists said, adding that, if anything, the extremists seemed more determined to seize the area, which would deepen their control over territory stretching from the Turkish border, across Syria and to the western edge of Baghdad.

“Instead of pushing them back, now every time they hear the planes, they shell more,” Ahmad Sheikho, an activist operating along the Syria-Turkey border, said of the Islamic State fighters. He estimated he heard a rocket explosion every 15 minutes or so.

Three mortar shells landed in a field in nearby Turkey, the Turkish military said in a statement. After the strike, Turkey’s military moved tanks away from the army post in the area, positioning them on a hill overlooking the border.

The push by Islamic State fighters caused thousands more Kurds to flee the Kobani area Monday, adding to about 150,000 refugees who have fled to Turkey since mid-September, one of the largest influxes of Syrian refugees since the war began 3½ years ago.

Washington and its Arab allies opened the air assault against the extremist group on Sept. 23, striking military facilities, training camps, heavy weapons and oil installations. The campaign expands on the airstrikes the United States has been conducting against the militants in Iraq since early August.

On Monday, the U.S.-led coalition carried out eight airstrikes targeting towns and villages in northern and eastern Syria controlled by the militants.

One strike hit a grain silo in the northern town of Manbij, setting it ablaze and killing two civilians working there, according to the Observatory’s director, Rami Abdurrahman.

“There was no ISIS inside,” Abdurrahman said, using an alternative name for the Islamic State group. The airstrikes, he said, “destroyed the food that was stored there.”

The U.S. Central Command said the silo was used by the militants “as a logistics hub and vehicle staging facility.”