By Phil Stewart and Warren Strobel

WASHINGTON (Reuters) - Since Turkey shot down a Russian fighter jet last week, the United States has quietly put on hold a long-standing request for its NATO ally to play a more active role in the U.S.-led air war against Islamic State.

The move, disclosed to Reuters by a U.S. official, is aimed at allowing just enough time for heightened Turkey-Russia tensions to ease. Turkey has not flown any coalition air missions in Syria against Islamic State since the Nov. 24 incident, two U.S. officials said.

The pause is the latest complication over Turkey's role to have tested the patience of U.S. war planners, who want a more assertive Turkish contribution -- particularly in securing a section of border with Syria that is seen as a crucial supply route for Islamic State.

As Britain starts strikes in Syria and France ramps up its role in the wake of last month's attacks on Paris by the extremist group, U.S. Defense Secretary Ash Carter publicly appealed this week for a greater Turkish military role.

The top U.S. priority is for Turkey to secure its southern border with Syria, the first official said. U.S. concern is focused on a roughly 60-mile (98-km) stretch used by Islamic State to shuttle foreign fighters and illicit trade back and forth.

But the United States also wants to see more Turkish air strikes devoted to Islamic State, even as Washington firmly supports Ankara's strikes against Turkey's Kurdistan Workers Party (PKK), viewed by both countries as a terrorist group.

Carter told a congressional hearing this week that most Turkish air operations have been targeted at the PKK rather than at Islamic State, but U.S. officials acknowledge some promising signs from Turkey, including moves to secure key border crossings.

For example, Turkish F-16 fighter jets last month joined an air operation to support Syrian rebels taking back two villages from Islamic State along the so-called Mara Line, a senior Obama administration official told reporters, speaking on condition of anonymity.

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The United States does not give data on the number or type of missions conducted by Turkish air force flights in Syria.

Turkey rejects any suggestion it is not playing its part in the fight against ISIL.

"We have taken part in at least half of the operations," a senior Turkish official told Reuters. "Apart from that, Turkey takes part in identifying targets and providing logistics and bases. We are in close cooperation with the U.S."

Russian President Vladimir Putin branded Turkey's shoot-down a war crime on Thursday and said Turkey would face further sanctions. Moscow has already banned some Turkish food imports as part of a wider package of retaliatory sanctions.

The United States hopes that tensions between Moscow and Ankara will ease quickly, allowing Turkey to take a more prominent role inside the U.S.-led coalition's air campaign, the first official said.

The Pentagon declined to comment on the status of Turkish flights since the shoot-down. Two Turkish officials declined to directly comment but stressed that Turkey remained part of the air coalition.

"For us nothing has changed," a senior Turkish official told Reuters.

U.S. officials stressed that overall coalition air operations had been unaffected by the tensions between Turkey and Russia.

There is debate within the Obama administration on how hard to push Turkey. U.S. officials broadly acknowledge its support has been vital to the U.S.-led campaign in Syria, allowing the coalition to stage strike missions out of a Turkish air base.

Turkey, for its part, has grown frustrated over the past few years at what it sees as indecision on the part of the United States and its Western allies, arguing that only Syrian President Bashar al-Assad's removal from power can bring lasting peace.

(Additional reporting by Nick Tattersall in Istanbul and Jonathan Landay in Washington; editing by Stuart Grudgings.)