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As U.S. fighter jets pound Islamic State targets in Syria, Washington's coalition allies appear increasingly absent from the air war, Reuters reports.

Although President Barack Obama's administration announced the Syrian air strikes three months ago as a joint campaign by Washington and its Arab allies, nearly 97 percent of the strikes in December have been carried out by the United States alone, according to U.S. military data provided to Reuters.

The data shows that U.S. allies have carried out just two air strikes in Syria in the first half of December, compared with 62 by the United States.

That accentuates a shift that began shortly after the start of the campaign in late September, when U.S. allies carried out 38 percent of the strikes. The percentage quickly dropped to around 8 percent in October and 9 percent in November, according to Reuters calculations based on the data.

U.S. officials are keen to prevent the coalition from fraying over concerns about the air campaign's direction. Some allies have long worried the air strikes might unintentionally bolster Syrian President Bashar al-Assad by striking a common enemy, sources said. Others in the region are also saying privately that the U.S.-led campaign against Sunni extremists needs to do more to help Sunni Muslims.

However, officials in the United States and the region insist that political tensions simmering within the coalition had nothing to do with dwindling coalition strikes.

"It's a question of targets. From a military perspective, the cooperation is extensive and deep," said a source familiar with Gulf strategy in the coalition.

Two factors are at play: a decline in the overall pace of strikes and fewer easier-to-hit fixed Islamic State targets after nearly three months of bombings, U.S. officials and Gulf sources say.

Such fixed targets were initially bombed by Bahrain, Jordan, Saudi Arabia and United Arab Emirates while the United States has from the start focused on more difficult ones, using precision-guided munitions to avoid civilian casualties.

"There are simply less (fixed) targets," said a U.S. official, speaking on condition of anonymity. "From our point of view, that's a good thing. It means that the strikes are having an impact."

Just under half of the 65 non-U.S. coalition air strikes in Syria tallied until 3 a.m. on Dec. 15 took place in the first nine days of the air campaign in late September, according to U.S. military data. U.S. allies carried out 20 air strikes in October and just 14 in November.

The only two strikes by Washington's allies this month targeted an electronic warfare garrison near the city of Raqqa on Dec. 7, a U.S. official said.

The drop in air strikes by coalition partners in Syria underscores the contrast with the campaign in Iraq.

Across the border, the United States has allies with highly trained and equipped air forces, including Britain, France, Canada and Australia. They see the air campaign in Iraq on far more solid legal ground, since they are there at the invitation of Baghdad.

Syria, on the other hand, is considered off-limits by many allies, particularly those in Europe, because of the Syrian government's public opposition to the U.S.-led air strikes.

"It's legal issues. It's concerns that our European partners and others have about where Syria is going," one U.S. official said. "So the reality is, even though we say the problem knows no border, by definition there's a distinction."

The United States intensified its campaign in Syria in October, carrying out 233 strikes, as the battle over the Kurdish border town of Kobani became a focal point. It carried out another 146 in November.

In total, the United States carried out 488 air strikes in Syria through Dec. 15, according to U.S. military data.