Tripoli, Libya (CNN) -- NATO officials have scheduled a Friday morning news conference to address concerns that airstrikes may have mistakenly killed four people in Libya who were rebel fighters and civilians the operation is supposed to protect.

Aircraft fired missiles on a rebel formation between al-Brega and Ajdabiya on the eastern Libyan battlefront, witnesses told CNN Thursday, an act that left the opposition wondering whether NATO aircraft conducted mistaken airstrikes.

It was unclear whether Libyan aircraft or NATO fired the missiles, but there haven't been Libyan air force planes in the skies for some time because NATO aircraft have established a no-fly zone.

NATO said it is investigating the incident Thursday and a spokeswoman was scheduled to address the media Friday morning.

The airstrikes killed two rebel fighters and two doctors. Also fourteen people were injured and six were missing, said Gen. Abdul Fattah Yunis, commander of rebel forces.

The airstrikes also angered opposition leaders, who say they are not making gains against Moammar Gadhafi despite NATO's no-fly zone and air cover.

Moods flashed from sorrow to anger quickly at a nearby hospital were the wounded were taken and many gathered.

These fighters, already demoralized by the superior firepower of the Libyan army, were convinced that missiles had come from a NATO plane.

One fighter screamed: "NATO, NATO, NATO, They shouldn't hit the revolutionaries. We're helpless."

Ahmed Abu Bakr, a doctor who came to Libya from Germany to volunteer, said he never that he would be patching up the wounded from friendly fire.

"I am very unhappy," he said. "They came here to help us not injure us."

And this may be the second NATO airstrike in less than a week that killed and wounded men on the wrong side, rebels say.

Last week, airstrikes hit rebel vehicles and killed at least 13 rebel fighters in the al-Brega area, a spokesman for the Libyan opposition said. NATO is investigating that airstrike also.

Weather conditions and tactics by the Gadhafi regime, such as using human shields and hiding equipment in populated areas, have hindered its efforts to launch airstrikes, NATO has said.

Yunis, the commander of rebel forces, called the airstrike an "unfortunate setback."

Planes fired on 20 rebel tanks near al-Brega Thursday morning, Yunis said. Gadhafi's troops moved forward after the attack, causing the opposition to pull back. Several of the tanks were destroyed.

The general said he wanted some answers from NATO on whether it or Gadhafi planes made the strike. If the latter, Yunis said, NATO should have prevented that by enforcing the no-fly zone.

He added that the rebels had notified NATO of the tank movement and of their presence.

"There is no tension between us and NATO; this is a war situation and we understand that mistakes are made," Yunis said.

A few hours after the strikes, civilians and rebels, fearing an approach by pro-Gadhafi forces, retreated from Ajdabiya, with hundreds of civilian cars and trucks loaded with rocket launchers and ammunition headed out of town in the direction of the opposition headquarters in Benghazi.

The strikes come amid a deadly stalemate between pro-Moammar Gadhafi forces and rebel fighters, diplomatic maneuvering to end the conflict. As the battle continues some wonder if the outgunned rebel forces can prevail and if NATO has the correct strategy to help.

Robert Baer, a former CIA operative, said NATO will have to put troops on the ground to assist the planes making airstrikes.

"First of all, the no-fly zone is not working," Baer said on "AC360" "Not a surprise to me that the NATO bombed the rebels force. We sort of got one foot in this but not completely. The logic of this conflict is you have to put people on the ground."

On the diplomatic front, a former U.S. lawmaker who has been trying to meet with Gadhafi told CNN "The Situation Room" that Friday will be his last opportunity.

Curt Weldon has said he will tell the leader to step aside and take other measures to end the bloodshed.

A former Republican U.S. House member from suburban Philadelphia, Weldon has been to Libya before in his work as a congressman. He said he came to the country this time with "a small private delegation."

Weldon said he has met with other Libyan officials, including the prime minster and Gadhafi's son, Saadi, conveying the Obama administration's stance on the crisis and reinforcing the importance of an immediate cease-fire monitored by the United Nations to protect civilians.

"I'm here only because I want to avoid war," Weldon told CNN's Wolf Blitzer. "I don't want to see American soldiers killed, and I don't want to see more innocent Libyans killed."

The World Food Programme said one of its humanitarian vessels loaded with food, medical supplies and doctors has reached the port of Misrata, providing what it called a "lifeline" for trapped civilians.

Misrata has been one the hardest hit cities during he struggle.

"This is a breakthrough for the U.N. humanitarian operation in Libya and allows us to reach tens of thousands of people who are caught in one of the fiercest areas of conflict," said WFP Executive Director Josette Sheeran. "It is vital that we get these relief supplies to the vulnerable."

CNN's Nic Robertson and Ben Wedeman contributed to this report

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