By Tom Perry

BEIRUT (Reuters) - Islamic State is preparing for a possible attack on a city in northeastern Syria near the border with Iraq where it remains a big threat despite recent setbacks, a Kurdish official told Reuters on Tuesday.

Hasaka province in northeastern Syria is strategically important for all sides and abuts Islamic State-held territory in Iraq, where the group is back on the offensive after losing the city of Tikrit at the start of the month.

The Syrian Kurdish YPG militia has recorded significant victories against Islamic State this year, driving it from the town of Kobani at the Turkish border and then taking two towns in Hasaka province with the help of a U.S.-led air campaign.

But Islamic State remains a danger, said Redur Xelil, YPG spokesman. Its targets include the provincial capital, Hasaka city, and the town of Tel Tamr, to the northwest. Islamic State is still believed to be holding some 200 Assyrian Christians abducted in February from villages near Tel Tamr.

"South of Hasaka there are areas that Daesh controls entirely. There is a big Daesh mobilization outside the city, and there are big fears of an attack on Hasaka city," Xelil said in an interview from the city of Qamishli via Skype.

Daesh is an Arabic name for Islamic State.

For now, the Islamic State priority is Tel Tamr, where it aims to cut a YPG supply route, he added. Islamic State was "trying to take big cities, to take the battle into cities" to mark it harder for the U.S.-led alliance to hit it, he said.

Hasaka is home to many Syrians who have fled areas further west, including the country's second city Aleppo, he added.

The Syrian Observatory, which monitors the Syrian civil war, reports daily clashes between the YPG and Islamic State fighters near Tel Tamr, and clashes between the Syrian military and Islamic State in areas west and east of Hasaka city.

AIR STRIKES

The YPG has emerged as the only partner for the U.S.-led alliance bombing Islamic State in Syria. But its effectiveness is greatly diminished beyond areas where the Kurds have set up autonomous zones since Syria's conflict erupted in 2011.

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The United States, rejecting any partnership with President Bashar al-Assad against Islamic State, is about to launch a program to train and equip members of the mainstream Syrian opposition in order to fight the jihadists elsewhere.

Xelil said the YPG had not been consulted on the program, adding that its requests for military supplies remained unmet.

Support for the YPG is a complicated issue for Western states because of the concerns of NATO member Turkey, which is worried about separatism among its own Kurdish population.

Since driving Islamic State from Kobani in January, the YPG has secured a large area around the town, including villages within the provincial boundaries of Raqqa - Islamic State's de facto capital.

Islamic State is the single biggest insurgent group in Syria, controlling areas in the east and the north.

In recent weeks, it has been mounting attacks well beyond those strongholds, targeting both insurgent- and government-held areas closer to Syria's main cities in the west.

Xelil said Islamist State had adapted to the air strikes. They were digging trenches and moving fighters and equipment in small convoys.

"They have found ways to deal with this situation, moving from area to area in a hidden way, or at times when there are no planes, either surveillance or bombers," he said."

"The Daesh threat will continue, all the while it dominates wide areas where it has supporters."

(Writing by Tom Perry; Editing by Crispian Balmer)