(CNN) -- An Iranian fighter jet targeted an unarmed U.S. Predator drone over the Persian Gulf this week, the Pentagon says.

It was the latest Iranian move aimed at thwarting American military airborne intelligence efforts in the region.

Defense Department spokesman George Little said on Thursday the unmanned MQ-1 drone was conducting routine classified surveillance over international waters on Tuesday when approached by an Iranian F-4.

The two aircraft came within 16 miles of each other.

The American drone was escorted by two U.S. military aircraft, which remained over international waters, Little said, adding that the Iranian jet departed after a verbal warning.

No shots were fired. However, Obama administration officials told CNN that the United States is concerned with Iranian intentions in incidents like this and whether such episodes could unintentionally trigger hostilities.

In December 2011, a highly sensitive U.S. Sentinel drone gathering intelligence on Iran's nuclear program for the CIA was captured by Iran after it crashed. In November, an Iranian warplane fired on a Predator over the Gulf.

After the November incident, the United States told Tehran that it would continue surveillance flights over international waters "consistent with longstanding practice," Little said.

"We also communicated that we reserve the right to protect our military assets as well as our forces and will continue to do so going forward," he said.

The use of U.S. drones so close to Iran remains a highly sensitive issue for American military intelligence.

The aircraft are capable of using sensors to gather information about Iranian military movements.