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The U.S. Navy is ready to confront any act of aggression, a top U.S. military official said Sunday after Iran began amassing a fleet of small boats that could launch suicide attacks in the Persian Gulf. But the Navy isn't the only sea-faring U.S. forces on guard in the Gulf.

The U.S. Coast Guard regularly patrols the waters near Iran and Iraq's coasts, but recent widely publicized operations by the Guard in the Gulf surprised many Americans despite its being in the region for nearly a decade.

"When most people think of the branches of the military, they think of the Army, Air Force, Navy, Marine Corps," said Lt. Joseph "Grant" Thomas, who commanded a U.S. Coast Guard ship, the cutter "Monomoy," in the Persian Gulf from 2009 to 2010.

The Monomoy rescued six Iranian fishermen from their sinking boat on Jan. 10.

"I don't think a lot of people necessarily understand the unique nature of the Coast Guard and how we are both a military and a federal law enforcement agency," Thomas said in an interview with Fox News.

Last month, U.S. Coast Guard Commandant Adm. Robert Papp acknowledged the frequent head-scratching over the Guard's role in the region.

"(I get) a lot of questions: Why is the U.S. Coast Guard there?" Papp said at a Navy symposium Jan. 12.

Six 110-foot Coast Guard cutters like the Monomoy have been deployed in the Middle East since 2003. Papp told Fox News in an interview that the Department of Defense "put in a request" for Coast Guard forces in 2003 when the U.S. Navy and other military branches "did not have sufficient patrol boats" for some efforts in the Middle East.

Thanks to their relatively small size, Coast Guard vessels can access waters that Navy vessels can't, Thomas said.

While in the Middle East, the Coast Guard operates under traditional military authorities and customary international law, which allows military personnel to visit, board and search vessels to confirm their identity, according to Papp.

The Coast Guard's primary mission was to protect the lifeblood of Iraq's fledgling economy: two Iraqi oil platforms, one of which is in waters claimed by Iran. Iraq produces millions of barrels of oil each day. But the U.S. Coast Guard has "backed away" a bit from protecting the Iraqi oil platforms, Papp said, as Coast Guard members train their Iraqi counterparts to take over.

Nevertheless, Papp said it's "fortunate" his forces "had a presence out there" when the six Iranian fishermen needed help, noting that a small Coast Guard vessel can saves lives just like a large Navy battleship. Five days before the Monomoy rescue, sailors from the John C. Stennis aircraft carrier group saved 13 other Iranians from a pirate attack.

"The Stennis battle group saved 13, a 110-foot patrol boat saved six," Papp said. "We might not be as big as the Stennis battle group, but we have a pretty good punch there."

In addition to protecting Iraq's oil platforms, the Coast Guard's mission includes escorting large Navy ships in the Persian Gulf to keep small boats away, boarding and investigating foreign ships based on intelligence or suspicious activity and routinely conducting "interaction patrols" to build bridges with local fishermen.

Most fishermen in the Persian Gulf welcomed the U.S. presence, feeling "a sense of security with having us within the region," said Thomas, who is now commanding officer of the Coast Guard cutter "Block Island" based out of Atlantic Beach, N.C.

During one "interaction patrol," the fishermen "were very excited and eager to see (us), and (we) ended up staying aboard for a while," Thomas recalled.

Thomas said saving lives on the water is "extremely rewarding," particularly when the Coast Guard can help mariners from a country that has "tense relations" with the United States.

But that doesn't mean the sea-going mission isn't dangerous.

"For the most part, I don't think it'd be any different than a soldier being on the ground over in Iraq or Afghanistan," Thomas said. "You just know that you're in a theater of operations."

In 2004, a Coast Guard member and two Navy sailors were killed when an explosives-laden boat targeted their ship protecting an Iraqi oil terminal. The loss of Petty Officer Nathan Bruckenthal was the Coast Guard's first combat death since the Vietnam War.

"That was constantly in the back of our minds as a potential risk as we were conducting (our) mission," Thomas said.

While Thomas was in the Persian Gulf, he said there were a couple of "tense moments" after threats to Iraq's oil platforms. Thomas said he "set battle stations" aboard his ship," but "fortunately those were short-lived and few and far between."

On Sunday, Navy Vice Admiral Mark Fox told reporters that Iran has increased the number of submarines to 10 and added "fast attack craft" in the Gulf. The deployment follows threats by Iran in recent weeks to disrupt shipping or strike U.S. forces in retaliation if its oil trade is shut down by sanctions or its nuclear program comes under attack.

"Some of the small boats have been outfitted with a large warhead that could be used as a suicide explosive device. The Iranians have a large mine inventory," Fox said.

While the U.S. naval forces are overwhelmingly superior, Iran still could make it "extremely difficult" for the U.S. Navy, he added.

"If we did nothing and they were able to operate without being inhibited, yeah they could close it, but I can't see that we would ever be in that position," Fox said.

But should tensions with Iran escalate on the high seas, Papp said his forces will likely stay out of it.

"In reality we fill a niche over there," he said. "Our cutters are lightly armed. They can protect themselves to a certain extent, but if something large was to happen over there, the U.S. Navy is quite prepared to take care of it."