The U.S. military picture in Syria is getting more chaotic and complicated by the day, putting new strains on the Obama administration's strategy of partnering with a hodgepodge of local fighters against the Islamic State group without getting pulled deeper into Syria's civil war or rupturing relations with Turkey.

Developments in recent weeks illustrate the fine balance the U.S. is trying to strike. For example, the Pentagon may get drawn into cooperating with Russian forces in Syria even though it believes Moscow's military intervention has only undermined the U.S. goal of defeating IS. And just last week the U.S. was compelled to respond when Syrian warplanes struck in an area not far from where U.S. troops were operating on the ground.

Adding to the volatile mix Tuesday, Turkish forces allied with Syrian Arab rebels and backed by U.S. air power pushed into Syria to retake Jarablus, a border town held by the Islamic State. In addition to helping with intelligence and aerial surveillance, the U.S. conducted airstrikes with A-10 and F-16 planes.

This is significant on several levels. First, it marks Turkey's most overt incursion into Syria. It also put Turkey on a path toward potential confrontation with Kurdish fighters in Syria who the United States is supporting in their fight against Islamic State militants and have been the most effective force battling IS in northern Syria.

U.S. officials were aware of a Turkish fear that a group of those U.S.-backed Kurdish forces, fighting as members of the Syrian Democratic Forces, might be preparing for a "jail break" from their American advisers to the Turkish border, according to a senior administration official traveling with Vice President Joe Biden, who was visiting Ankara on Tuesday.

A few days ago the Turks fired artillery across the border as warnings to the Kurdish fighters. "That's a big problem" because the U.S. does not want a direct clash between the Turks and the Kurds, the U.S. official said, speaking on condition of anonymity because the official wasn't authorized to describe U.S. military and diplomatic strategy.

Biden told reporters that the U.S. backed Turkey's demand for limits on Kurdish expansion. He said Kurdish forces "must move back. They cannot, will not, under any circumstance get American support if they do not keep that commitment."

Turkey is particularly sensitive to Kurdish advances in northern Syria because the Turks have a long-running fight against Kurdish insurgents on their side of the border. That is why Turkey has been upset with U.S. empowerment of a Kurdish militia known as the YPG, whom the Turks consider to be terrorists. They are the predominant element in the umbrella group the U.S. has created and called the Syrian Democratic Forces.

The U.S.-Turkish relationship has been under severe strain since a failed military coup in July. Turkey accuses a U.S.-based Muslim cleric, Fethullah Gulen, of masterminding the attempted putsch. Gulen has denied any involvement, but Turkey has demanded his extradition from the U.S. In addition to being a longtime U.S. ally in NATO, Turkey has allowed the U.S. to fly attack missions against IS from Incirlik air base in southern Turkey.

These and other complexities in Syria have been present almost from the start of U.S. military involvement there in September 2014, when President Barack Obama authorized airstrikes aimed at degrading and defeating the Islamic State group, which has made the Syrian city of Raqqa its defacto capital. At that point, however, the Pentagon had no troops on the ground in Syria. Today it has upward of 300 there working with local fighters.

"The complications have come to a head," said Jennifer Cafarella, a Syria analyst at the Institute for the Study of War. "They have always been a factor in how the U.S. navigates our anti-ISIS strategy in Syria," but the Turks changed the calculus by committing special forces and conventional troops to support the recapture of Jarablus, the IS-controlled Syrian border down on the Euphrates River.

By doing so, the Turks ensured the U.S. would not employ the Kurd-dominated Syrian Democratic Forces to seize Jarablus, Cafarella said.

That is not necessarily a bad thing, she added. It could help solve a problem the U.S. created by aligning itself so closely, and enabling so successfully, the Syrian Democratic Forces. It gives at least the appearance of a more even-handed U.S. approach to the Kurds and the Arabs, she said.

"This could be a good development," from the U.S. standpoint, she said.

Russia may not see it that way. The Foreign Ministry in Moscow issued a statement expressing deep concern about Turkey's ground incursion, saying it raises the risk of civilian casualties and the worsening of ethnic tensions between Kurds and Arabs.