U.S. officials have now spotted a Russia drone and several manned flights by Russian pilots familiarizing themselves with terrain over western Syria, two U.S. defense officials told The Daily Beast on Thursday.

The flights, around the western Syrian province of Idlib, suggests that Russia’s initial mission will focus on attacking areas controlled by regime opponents other than the self-proclaimed Islamic State. The flights are also near Latakia, where Russia has a base and has stationed fighter jets, roughly 500 troops, nine T-90 tanks, and enough modular housing for 2,000 troops, according to U.S. military assessments.

“We still don’t know what their intent is,” one U.S. defense official told The Daily Beast. The flight activities “suggests they are planning to conduct some kind of air operation.”

Others defense officials believe the flights are designed to protect bases belonging to Syrian President Bashar al-Assad’s regime.

In addition to the surveillance drones, Russia has “Fencer” advanced-attack aircraft jets and “Frogfoot” jets for close air support among its arsenal, according to U.S. estimates.

The presence of both manned and unmanned aircrafts marks the most aggressive military maneuver by President Vladimir Putin’s forces since their buildup in Syria began several weeks ago. Russian military commanders are also reportedly coordinating with Syrian and Iranian officials in Baghdad—the first such report of Russian activity in Iraq during the ISIS war. The accelerated build-up comes days before the United Nations General Assembly convenes in New York.

At a briefing Thursday with Ukraine's Minister of Defense, Colonel-General Stepan Poltorak, Secretary of Defense Ash Carter would not say what the U.S. response would be should Russia strike non-ISIS opponents to the Assad regime.

Carter also would not say whether he would talk to his Russian counterpart about how to deconflict Russian and U.S.-led coalition flights over Syria.

Rather he suggested the U.S. was waiting to see if—and how—Russia conducted military operations in Syria.

Russia's "intentions will be revealed in the actions they take," Carter told reporters.

U.S. officials had told The Daily Beast this week that they expected Russian military activity to begin soon as 28 combat planes, 26 helicopters, and other various equipment began pouring into Syria.

The possible escalated Russian intervention on behalf of the Assad regime could bolster Assad’s military standing. In recent weeks, Assad troops have lost parts of Aleppo and Idlib provinces, potentially leading Russia to bolster its presence in the conflict.

But some U.S. officials worry that Russia’s intervention could weaken the U.S. ability to shape the outcome in the year-long war against ISIS.