American warplanes and combat advisers are once again backing Turkish military units battling Islamic State fighters in northern Syria, the most tangible indication yet that months of tension between the NATO allies may be easing. 

Four airstrikes, occurring Tuesday near the town of al Bab, hit Islamic State construction equipment and tactical vehicles, including an armored personnel carrier, said Air Force Col. John Dorrian, a spokesman for the U.S.-led coalition based in Baghdad. Dorrian indicated the mission, developed with Turkish commanders, most likely was enabled by American ground troops working nearby.

"One of the things that we very much prize is to have boots on the ground operating in proximity to, or directly with, our partner forces. That's the preferred alternative," Dorrian said, when asked whether airstrikes conducted in support of Turkey require spotting by American air controllers who can see the desired targets. "And that is one of the reasons why we have to be very careful, especially on a very crowded and complex battlefield with a lot of different actors to make sure that we're hitting the targets that we intend to hit."

It's not immediately clear what sort of aircraft carried out the airstrikes or what type of weapons were used. U.S. military officials did not immediately provide a response to those questions.

U.S. forces are  likely to remain active in and around al Bab during the days ahead, Dorrian said, calling Tuesday's operation "in our mutual interest" with Turkey.

"This is something we expect to continue doing," he added.

There are about 500 U.S. troops inside Syria, Dorrian said. They compose a mix of special operations forces and other elite personnel focused on training and advising the patchwork of American allies — to include Turkey — fighting the Islamic State there.

Turkish forces have suffered significant casualties while battling ISIS militants entrenched in al Bab, located some 20 miles south of Turkey's border with Syria. The airstrikes there occurred just days after U.S. aircraft, following a lengthy drought, resumed show-of-force and surveillance flights in the area.

"They're fighting a determined enemy that has no problem with using civilian shields and they've had quite some time to dig into the areas where they are," Dorrian said of the resistance experienced by Turkish forces there. "So there's gonna be tough fighting there. It's not surprising. ... They have had a lot of time to build their defenses in all the areas that they control, and so they're not going to be easily dislodged."

Launched in November despite objections from the Pentagon, Turkey's offensive in al Bab began as the U.S. sought to galvanize support for its effort to route Islamic State fighters from Raqqa, the group's de facto capital about 125 miles to the east. The U.S. stopped providing the Turks with air support and ground-level military advisers after Turkey refused to join that effort. The standoff sparked concern that Turkey might curtail the Americans' access to Incirlik Air Base, a major hub for the air campaign in Iraq and Syria.

With the resumption of U.S. support for Turkey, it remains to be seen whether Turkey, in turn, will prioritize any of its troops and weapons for the Raqqa fight. The Americans' most capable allies there are members of a Syrian Kurdish militia, forces Turkey considers associates of terror groups responsible for attacks on Turkish soil.

American officials have sought to downplay the tension. Meanwhile, Russian military aircraft have, in lieu of U.S. support, began hitting Islamic State targets in the area. The U.S. military's top general, Joint Chiefs Chairman Joseph Dunford, suggested last week that the Russians carried out those airstrikes independently. Turkish officials, however, have indicted they had no choice but to seek Russia's help.

Dorrian indicated Tuesday that U.S. commanders are talking with their Russian counterparts about operations in and around al Bab. "We do maintain our channel for deconfliction with Russia," he said. "That's a regular dialog."