U.S. special operations forces mounted an unsuccessful mission to rescue two American University of Afghanistan professors kidnapped in Kabul last month -- after an earlier mission was aborted when the White House withheld its approval -- defense officials with knowledge of the incident told Fox News.

Fox News is told the operation, which took place a few days after their Aug. 7 kidnapping, killed seven enemy fighters. But when the firefight ended, there was no sign of the hostages.

One of the hostages is American; the other is Australian.  

It was not the first attempt by the U.S. military to rescue the two professors.

The first mission was aborted when the American rescue force could not obtain White House approval for the mission amid concerns about the intelligence, according to three defense officials. They were in the air heading to the target area at the time and had to return to their base in Afghanistan. 

When approval was granted by President Obama the next day, the American assault force headed back to the area where they believed the two hostages were being held and engaged an enemy force at a compound believed to contain the hostages.

“It didn’t go the way it was supposed to go,” said one official. 

The operation took place near Jalalabad in eastern Afghanistan. It is not immediately clear if the hostages were ever inside the compound or moved before the second rescue attempt could take place, according to one official.

The initial mission was cancelled because the intelligence was not completely firm and there was not consensus among different government agencies, which prevented the White House from granting approval, one official told Fox News.

“They turned back that first night because they didn’t have authority. They could have gone without permission if they thought hostages' lives were in imminent danger,” according to one official.

The professors were kidnapped while driving in Kabul on Aug. 7.  The Haqqani network, an Afghan insurgent group, is suspected.

Today, the U.S. intelligence community is “not confident” where the two hostages are, according to multiple officials.

One source told Fox News the delay in approving the first rescue attempt was due to the White House “bureaucracy.”

“I have not heard any of that,” said another official when asked by Fox News for comment. “For this operation, we didn’t get the president’s authority in time because the intelligence was not firm.” 

“If the hostages are … in imminent danger, we go,” said the official. “That was not the case this time, so we went the next day.” 

Administration officials confirmed the hostages were not at the location.

“We will not provide further information on this mission in order to protect the safety of hostages and operational security,” Pentagon press secretary Peter Cook said in a statement, noting Obama authorized the mission at Defense Secretary Ash Carter’s recommendation. “Military hostage rescue operations are inherently sensitive and dangerous and careful deliberation went into this mission,” he added.

A senior administration official also said Obama approved the mission “soon after the Pentagon submitted their request.” 

“The president's swift approval provided the U.S. military the authorization to conduct the operation in Afghanistan,” the official said. “When the operation was launched, we believed the hostages were present at the location but unfortunately, they were not. The U.S. government's hostage recovery enterprise continues to work towards ensuring the hostages' safe recovery.”

None of the U.S. special operations forces were killed or wounded during the exchange of gunfire with the enemy force.  

In addition to the kidnapping of the two professors last month, American University in Kabul was attacked separately when gunmen and a suicide bomber attacked the school. The Taliban are suspected of carrying out the attack which killed at least 12.

Lucas Tomlinson is the Pentagon and State Department producer for Fox News Channel. You can follow him on Twitter: @LucasFoxNews