Tripoli, Libya (CNN) -- Airstrikes killed four people in eastern Libya on Thursday, an opposition general said, and rebel fighters and civilians made a wild, panicky retreat from a major city.
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 on the forces they are trying to protect.
It's 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's looking into the strikes but didn't say who carried it out. In a statement, it said that hostilities between al-Brega and Ajdabiya have "been fierce for several days. The situation is unclear and fluid with mechanized weapons traveling in all directions."
"What remains clear is that NATO will continue to uphold the U.N. mandate and strike forces that can potentially cause harm to the civilian population of Libya," NATO said.
Gen. Abdul Fattah Yunis, commander of rebel forces, told reporters of the "unfortunate setback."
The 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.
Two soldiers and two medics died in the airstrike. Fourteen people were injured and six are missing, Yunis said.
Witnesses said the rebel fighters saw an airplane and thought it was from NATO, CNN's Ben Wedeman reported. Later, the plane returned and fired several missiles and then made another strike. Ambulances responded, and the injured people were taken to a hospital in Ajdabiya.
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. Tanks were reportedly part of the column of vehicles.
The Gadhafi regime's persistence in the face of NATO forces has frustrated and angered opposition leaders, who say they are not getting traction despite the alliance's no-fly zone and air cover. NATO said weather conditions and tactics by the Gadhafi regime, such as using human shields and hiding equipment in populated areas, have hindered its efforts.
If the latest bombing run were a NATO strike, rebels said it would have been the second in a week's time. 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.
The strikes come amid a deadly stalemate between pro-Moammar Gadhafi forces and rebel fighters, diplomatic maneuvering to end the conflict and blunt doubts about the potency of the opposition forces.
Asked whether rebel fighters can push into Tripoli and oust Gadhafi, Gen. Carter Ham, U.S. Africa Command's chief, told senators at a hearing Thursday, "I would assess that as a low likelihood."
Also Thursday, a senior Libyan government official said four detained journalists are in government hands and are safe. They will be brought to Tripoli and released, but the timing is uncertain, the official said.
GlobalPost, an international news website, said pro-Gadhafi forces detained one of its freelancers and three other journalists. They had been taken Tuesday night on the outskirts of al-Brega, said Peter Bouckaert, director of emergencies for Human Rights Watch.
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," he added.
Fighting has raged across the country. Rebel fighters and pro-Gadhafi forces have been pushing back and forth between al-Brega and Ajdabiya. In the capital, Tripoli, four explosions were heard, and two aircraft could be seen overhead Thursday.
Libyan government spokesman Musa Ibrahim described the Thursday airstrikes as heavy and "really intensive." He said the strikes hit "military academies around Tripoli where students and teachers work."
A British airstrike hit an oil field in the eastern town of Sarir on Wednesday, causing damage to a main pipeline, Libyan Deputy Foreign Minister Khaled Kaim told reporters. But NATO's operation in Libya denied responsibility, saying such blame reflects the desperation of the Gadhafi regime.
"We are aware that pro-Gadhafi forces have attacked this area in recent days, which resulted in at least one fire at an oil facility north of Sarir," said Lt. Gen. Charles Bouchard, commander of NATO's Operation Unified Protector.
"We have never conducted strike operations in this area because his forces were not threatening civilian population centers from there," Bouchard said. "The only one responsible for this fire is the Gadhafi regime, and we know he wants to disrupt oil getting to Tobruk."
Meanwhile, violence continues in the western city of Misrata. Ham said the "regime has a significantly degraded ability to continue to attack civilians" and Misrata is a "notable exception."
"The opposition forces have held an area in the northeastern portion of the city, in the port. And frankly, the port has been operating to get some relief," he said. "But the regime forces are and remain active in the city against civilians."
U.N. Secretary-General Ban Ki-moon has said conditions in Misrata are "especially grave," citing "reports of the use of heavy weapons to attack the city, where the population is trapped and unable, as a result of heavy shelling that has continued over several weeks, to receive basic supplies, including clean water, food and medicines."
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.
"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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