FILE - This undated file image posted on a militant website on Tuesday, Jan. 14, 2014 shows fighters from the al-Qaida linked Islamic State of Iraq and the Levant (ISIL) marching in Raqqa, Syria. Once a vibrant, mixed city considered a bastion of support for President Bashar Assad, the eastern city of Raqqa is now a shell of its former life, transformed by al-Qaida militants into the nucleus of the terror group's version of an Islamic caliphate they hope one day to establish in Syria and Iraq. In rare interviews with The Associated Press, residents and activists in Raqqa describe a city where fear prevails, music has been banned, Christians have to pay religious tax in return for protection and face-veiled women and pistol-wielding men in jihadi uniforms patrol the streets. (AP Photo/militant website, File)

* Captured Kurdish villages are near Turkish border

* Kurds in Turkey asked to go reinforce Syrian Kurdish city

* U.S. general sees role for Kurds against Islamic State

* Drone spotted over Islamic State territory (Adds Turkish prime minister's comment on refugees)

By Tom Perry and Laila Bassam

BEIRUT, Sept 18 (Reuters) - Islamic State fighters besieged a Kurdish city in northern Syria on Thursday after seizing 21 villages in a major assault, prompting a call to arms from Kurds in neighboring Turkey who urged followers to go and help resist the group's advance.

The attack on the city of Ayn al-Arab, known as Kobani in Kurdish, came two days after the top U.S. military officer said the Syrian opposition would probably need the help of the Syrian Kurds to defeat Islamic State.

With the United States planning to expand military action against Islamic State from Iraq to Syria, a surveillance drone was spotted 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 according to its radical vision of Sunni Islam.

The United States is conducting air strikes against Islamic State in Iraq and last month Obama authorized surveillance flights over Syria.

Islamic State fighters, armed with heavy weaponry including tanks, seized a group of villages near Kobani in an offensive which the Observatory said had started on Tuesday night.

It said 21 villages had fallen to Islamic State in the last 24 hours as the group advanced 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, giving the names of 28 members of a single family he said had been taken captive. It was not possible to verify his account immediately.

The Kurds were appealing for military aid from other Kurdish groups in the region including the Kurdistan Workers' Party (PKK), he said. Support from Kurds who crossed from Turkey helped to repel an Islamic State attack on Kobani in July.

Turkish PKK rebels later issued a call for young men in Turkey's southeast to join the fight in northern Syria.

"The youth of northern Kurdistan (southeast Turkey) should go to Kobani and take part in the historic, honorable resistance," the Kurdistan Workers Party (PKK) said in a statement on its website.

Footage posted on YouTube on Wednesday by the YPG, the main Kurdish armed group in Syria, appeared to show Kurdish fighters armed with assault rifles and rocket-propelled grenades battling a tank flying the Islamic State's black flag west of Kobani.

TANKS, ROCKETS, ARTILLERY

About 3,000 men, women and children arrived at the Turkish border roughly 10 km (6 miles) from Kobani but were still waiting on the Syrian side after night fell, a Reuters witness said. Turkish forces stopped the crowd from crossing.

Prime Minister Ahmet Davutoglu said the governors of border provinces in Turkey, where Kurdish militants have waged a three-decade insurgency to push for greater autonomy, had been ordered to extend aid to refugees on Syrian side of the border.

"We're ready to help our brothers who are building up at the borders regardless of their ethnicity, religion and sect. But our priority is to deliver aid within Syria's borders," he told reporters in Ankara.

Redur Xelil, spokesman for 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.

Western states have expanded contact with the main Syrian Kurdish party, the PYD, since Islamic State seized wide areas of Iraq in June. The YPG, which says it has 50,000 fighters, says it should be a natural partner in a coalition the United States is trying to assemble to fight Islamic State.

But the Syrian Kurds' relationship with the West is complicated by their ties to the PKK - a group listed as a terrorist organization in many Western states because of the militant campaign it waged for Kurdish rights in Turkey.

Western officials also cite concerns about the Syrian Kurds' ambiguous relationship with President Bashar al-Assad, who has mostly left the Kurds to their own devices while focusing its firepower on insurgents fighting to unseat him. The Syrian Kurds have denied accusations of cooperating with Damascus.

Obama's plans to expand support for groups fighting Islamic State in Syria have focused on Sunni Muslim insurgents deemed moderate by Washington.

But General Martin Dempsey, chairman of the U.S. military's Joint Chiefs of Staff, said on Tuesday that the Syrian opposition would probably need the help of the Syrian Kurds, together with the Jordanians and the Turks, to beat Islamic State in Syria.

There have recently been new signs of cooperation between the YPG and such non-Islamist insurgent groups in Aleppo province: they set up a joint operations room to direct the fight against Islamic State in that area.

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.

Since Obama authorized 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 of the Observatory. "They hadn't seen them before," he said. (Additional reporting by Daren Butler in Istanbul, Gulsen Solaker in Ankara and Seyhmus Cakan on the Turkey/Syria border; Editing by Janet Lawrence, Andrew Heavens and Dominic Evans)