Islamic State fighters captured villages and besieged a Kurdish city in northern Syria near the border with Turkey on Thursday in a major assault that prompted a commander to appeal for military aid from other Kurds in the region.
With the United States planning to expand military action against Islamic State from Iraq to Syria, a surveillance drone was spotted for the first time over nearby Islamic State-controlled territory in Aleppo province, the Syrian Observatory for Human Rights, which tracks Syria's civil war, said.
It was not immediately clear who was operating the drone.
U.S. President Barack Obama last week said he would not hesitate to strike the radical Islamist group that has used Syria as a base to advance its plan to reshape the Middle East.
The United States is conducting air strikes against Islamic State in Iraq and last month Obama authorized surveillance flights over Syria.
In an advance near the border with Turkey, Islamic State fighters using heavy weaponry including tanks seized a group of Kurdish villages near the city of Ayn al-Arab, also known as Kobani. The Observatory said 21 villages had fallen to Islamic State fighters advancing on the city.
"We've lost touch with many of the residents living in the villages that ISIS (Islamic State) seized," Ocalan Iso, deputy head of the Kurdish forces in Kobani, told Reuters via Skype.
He said the group was committing massacres and kidnapping women in the newly-seized areas. It was not possible to immediately verify his account.
The Kurds were appealing for military aid from other Kurdish groups in the region including the Kurdistan Workers' Party (PKK), he said. Support from Kurdish fighters who crossed from Turkey helped to repel an Islamic State attack on Kobani in July.
The Observatory said there were fears of massacres in the areas seized by Islamic State. "This is a very important advance for them," Rami Abdulrahman, the Observatory's founder, told Reuters, speaking by phone.
Tanks, rockets, artillery
Redur Xelil, spokesman for the main armed Kurdish group in Syria, the YPG, said Islamic State had encircled Kobani.
The group was using tanks, rockets and artillery in the attack. "We call on world powers to move to halt this barbaric assault by ISIS," he told Reuters via Skype.
Islamic State has been trying to establish control over a belt of territory near the border with Turkey, expanding out of its strongholds further east in the provinces of Raqqa and Deir al-Zor, which borders Iraq.
The group advanced westwards into northern Aleppo province in August, seizing territory from less well-armed groups that have been fighting President Bashar al-Assad's forces.
Since Obama authorised aerial surveillance over Syria, activists have reported drones in the skies over Raqqa, which is 400 km (250 miles), northeast of Damascus.
Residents had seen at least one drone over the Islamic State-controlled towns of al-Bab and Manbij in northeastern Aleppo province on Thursday, said Abdulrahman.
"They hadn't seen them before," he said. Islamic State had evacuated buildings it was using as offices in the area, he added.
It reflects the pattern in other Islamic State-controlled areas of Syria. In apparent anticipation of U.S. action, the group has evacuated bases and moved its fighters and heavy weaponry.
Al-Bab is 40 km northeast of Aleppo, which is a crucial theatre of the war between Assad and the insurgents. Assad's forces and his opponents are battling for control of the city.
The top U.S. military officer said on Tuesday the United States was planning "a persistent and sustainable campaign" against Islamic State in Syria. General Martin Dempsey, the chairman of the Joint Chiefs of Staff, said the United States was not preparing to unleash a "shock and awe" campaign of overwhelming air strikes in Syria.