BEIRUT -- 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 (five kilometers) 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 of 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 on Monday, adding to some 150,000 refugees who have fled to Turkey since mid-September, one of the largest influxes of Syrian refugees since the war began 3 years ago.
The Kurds were particularly fearful that the militants would repeat the mass killings of men and seizures of women that occurred in Iraq in August, after Islamic State fighters seized villages dominated by Iraqis of the Yazidi minority.
Men were leaving their families in Turkey and then heading back to Kobani to fight, Sheikho said.
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 upon the airstrikes the United States has been conducting against the militants in Iraq since early August.
The airstrikes are meant to ultimately destroy the group, which has declared a self-styled caliphate, or Islamic state, ruled by its harsh interpretation of Islamic law in areas under its control. Its brutal tactics, which include mass killings and beheadings, have galvanized the international community to take on the militants.
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."
Pentagon spokesman Col. Steve Warren said the U.S was investigating reports that civilians were killed in the strike, but had found nothing so far to corroborate the allegation. He acknowledged, however, that because of limitations of Pentagon intelligence in Syria, the U.S. could not be certain that every casualty of the coalition airstrikes was a combatant.
Another strike overnight hit the entrance of Syria's largest gas plant in the eastern Deir el-Zour province, but did not damage the facility itself, the Observatory said. U.S. Central Command said the strikes in Deir al-Zour hit two military vehicles.
More raids Monday morning struck the town of Tel Abyad on the Syria-Turkey border, according to a resident on the Turkish side of the frontier.
Mehmet Ozer, who witnessed the airstrikes from the Turkish side of the border, told the Associated Press the raids hit an abandoned military base and an empty school, sending smoke and dust into the air. He said militants evacuated the base about three months ago.
"They (the coalition) must not have fresh intelligence," Ozer said.
U.S. Central Command said the strikes targeted a compound and an airfield used by the Islamic State group.
The U.S.-led coalition includes Saudi Arabia, Bahrain, the United Arab Emirates, Qatar and Jordan. Several European countries also are contributing to U.S. efforts to strike the Islamic State group in Iraq, including France, the Netherlands, Denmark, Belgium and Britain.
The two purported civilian casualties in Manbij would add to the 19 civilians the Observatory says have already been killed in the coalition airstrikes.
On Sunday, Human Rights Watch said it had confirmed the deaths of at least seven civilians - two women and five children - from apparent U.S. missile strikes on Sept. 23 in the village of Kafr Derian in northeast Syria. The New York-based group said two men were also killed in the strikes, but they may have been militants.
"The United States and its allies in Syria should be taking all feasible precautions to avoid harming civilians," said Human Rights Watch official Nadim Houry.
"The U.S. government should investigate possible unlawful strikes that killed civilians, publicly report on them, and commit to appropriate redress measures in case of wrongdoing."
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Hadid reported from Beirut. Associated Press reporters Mohammed Rasool in Suruc, Turkey, Ryan Lucas in Beirut, Suzan Fraser in Ankara, Turkey, and Ken Dilanian in Washington contributed to this report.