WASHINGTON — A Russian military aircraft flew over a U.S. base in southern Syria on Friday near the country’s border with Iraq in what Pentagon officials said was an intelligence-gathering mission.

“They continue to undertake activities that are very concerning to us,” said a senior U.S. defense official who spoke on condition of anonymity. “This morning, they flew an intelligence-reconnaissance mission over al-Tanf garrison and were in that space for an extended period of time.”

The Russian aircraft conducted the surveillance at a time when there was a gap in U.S. coverage in the area, the official said, meaning no American aircraft were available to respond.

“You can never cover everywhere that you have to cover all at once,” he said. “This just happened to be one of the periods when we had a small gap in our coverage.”

Al-Tanf is an American military base located in a part of Syria that’s controlled by forces opposed to the regime of Syrian President Bashar al-Assad and aligned with the United States. The base houses American troops and partner forces. Roughly 900 U.S. service members are based in Syria to support operations against the Islamic State group and train partner forces.

The senior official also said Russian aircraft intercepted a U.S. MQ-9 drone on Friday, continuing a recent pattern of similar behavior. Last week, Russian fighter jets harassed other MQ-9s over Syria by dropping flares in their path. A Russian Su-27 fighter collided with an MQ-9 in March and forced it to crash in the Black Sea. After the drones were harassed by Russian planes last week, they succeeded in a strike that killed ISIS group leader Usamah al-Muhajir.

The senior defense official said the Russian plane Friday approached the MQ-9 in a “safe” but “unprofessional” manner. He said Moscow’s claim of a routine exercise was “just an excuse to go after our MQ-9s.”

Temporary increases in Russian military activity in the region are nothing new, the U.S. official said.

“What has been qualitatively different since March is that there’s been an increase in activity and an increase in the unprofessional nature of [Russia’s] interactions,” the official said. “[Before] they wouldn’t turn and attempt to dogfight our aircraft. That is almost routine now.”

During the March encounter, the Russian Su-27 flew so close to the American MQ-9 that it clipped the drone’s propeller. The incident was captured on the drone’s camera, and the Russian pilots involved were later given medals for causing the unmanned American aircraft to crash. Defense officials said they believe the increase in activity is part of a general effort to pressure the United States into leaving Syria, where it’s been running anti-terror operations for a decade. Assad’s regime is aligned with Russia and Iran.

“As Russia and Iran have pulled themselves closer together, there is interest on both sides to push us out of Syria. The Assad regime certainly wants us gone,” the senior defense official said. “There’s a confluence of interests between those three groups … to push the U.S. out.”