The Pentagon Monday declared "illegitimate" a series of photos circulating online which purport to show U.S. servicemen holding up handwritten signs objecting to war in Syria.

Five remarkably similar photos posted on a defaced U.S. Marine Corps website appeared to show American troops from the Army, Air Force, Navy and Marines concealing their faces with the messages, such as "Stay out of Syria," and, "I will not fight for Al Qaeda in Syria."

"We believe the photos are illegitimate," U.S. Marine Corps spokesman Lt. Col. Neil Murphy told ABC News on Monday.

Some opponents of the proposed U.S. military strike on Syria appear to have been taken in by the alleged hoax, including Texas GOP Sen. Ted Cruz.

Cruz cited one of the widely circulated photos on ABC News' "This Week" last Sunday when he defended previous remarks that a U.S. strike on Syria would amount to America playing the role of al Qaeda's air force.

Asked for the basis of his statement, Cruz told George Stephanopoulos, "What I can tell you is that actual line initially was said by [former Ohio Democratic Rep.] Dennis Kucinich. And where I saw it after that was a current naval sailor who tweeted and said, 'I didn't sign up to serve as al Qaeda's air force.'"

In the photo, a man wearing the uniform of a Navy chief petty officer holds a placard that blocks his face and read, "I didn't join the Navy to fight for al Qaeda in a Syrian civil war."

But the Marines' spokesman said the "sailor" likely wasn't an American at all.

"We do not know the identity of the person in the photo, however, we are confident that the photo is part of a disinformation campaign that was part of a targeted 'cyber redirect' last week," Murphy told ABC News.

Murphy was referring to a brief period when the Marines.com website was briefly hijacked by the Syrian Electronic Army (SEA), a pro-Assad hacking group responsible for a number of high-profile, if relatively unsophisticated, cyber attacks on major Western news organizations like The New York Times, The Washington Post and The Associated Press.

Cruz's office confirmed the tweet he mentioned on Sunday was of the same photo of the alleged U.S. Navy sailor who appeared on the hacked site.

"Our office doesn't generally fact check Tweets, but the Senator has heard similar sentiments in numerous interactions with veterans and his Texas constituents in recent days," Cruz spokesman Sean Rushton told ABC News late Monday.

During the cyber attack, computer users attempting to go to the Marines.com website would actually be taken to another site featuring several pictures of the alleged anti-intervention servicemen along with a statement from the SEA urging American troops to defy their orders should they be called to go to Syria.

"Obama is a traitor who wants to put your lives in danger to rescue al Qaeda insurgents," a message from the SEA on the site said, calling the Marines their "brothers."

In the complex conflict in Syria, the U.S. government has taken the side of the opposition, even though a major player in the rebellion is Jabhat al-Nusra, a Sunni Muslim extremist organization that has sworn its allegiance to al Qaeda. Obama and other top U.S. officials have said the proposed military action would not include American boots on the ground.

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In an email interview with ABC News following the Marines.com attack, an alleged leader of the SEA, who goes by SEA The Shadow online, said they posted the images because they "saw many soldiers… unwilling to join this war."

SEA The Shadow said Monday their group originally found the pictures on "American Facebook pages," but did not respond to a follow up request for links to those pages.

The alleged SEA leader previously told ABC News they had targeted the news websites earlier because the SEA believed they were spreading opposition lies about the Syrian conflict.

After the news attacks, cyber security experts told ABC News that the SEA has shown technical and planning capabilities beyond that of the average hacktivist, but doubted the group had the skill to inflict serious damage to the U.S. military, economy or homeland infrastructure.

Still, as a U.S. official said, the group is fully on the U.S. government's radar and is considered " more than just a nuisance."