* Militia factions fighting for past month
* Neither side believed to have an air force
* U.S., Egyptian, NATO officials deny involvement
* Libyan government armed forces largely helpless (Adds government reaction)
By Heba al-Shibani and Ahmed Elumami
TRIPOLI/BENGHAZI, Libya, Aug 18 (Reuters) - Unidentified warplanes bombed militia positions in the Libyan capital Tripoli on Monday, the government said, in what could mark a sharp escalation of fighting between armed factions that has thrown the North African state into anarchy.
Several Libyan TV channels said planes targeted bases of militiamen from Misrata who have been battling brigades from the western Zintan region to gain control of Tripoli in Libya's worst violence since Muammar Gaddafi was toppled in 2011.
Tripoli residents said they heard several jets flying after midnight, followed by loud explosions. No more planes were heard but fighting resumed in some parts of the city in the morning.
A Zintani source said fighters of his unit saw planes bombing a Misrata emplacement. "Our forces at the airport saw massive and accurate bombings (on a Misrata position)," he said. Reuters reporters were not immediately able to access the area.
Pro-Misrata news websites accused renegade General Khalifa Haftar of responsibility for the air strike. Haftar has previously used some aircraft from an air base under his control in Benghazi to attack militant Islamists in eastern Libya.
Haftar's spokesman declined to comment "at the moment" when Reuters asked whether his planes had attacked Tripoli targets.
The weak central government in Tripoli said it did not know to whom the warplanes belonged. "The government does currently not have any convincing indications to establish which side was behind this," it said in a statement.
The fighting hitherto was limited to ground action with artillery and rockets. None of the militias are believed to own warplanes, while the central government has only an outdated air force badly in need of repair.
Libyan news channels speculated that the country's neighbours might be behind the overnight air action.
A U.S. official and an Egyptian security source, both speaking on condition of anonymity, said their countries had not been involved. A NATO official said: "There are no fighter jets under NATO command involved in operations over Libya."
NATO air strikes helped rebels overthrow Gaddafi in 2011.
The central government of the OPEC member state has no functioning national army and almost no control over the capital, with most officials working from Tobruk in Libya's far east where the new parliament has set up to escape the violence.
An Egyptian security source said air traffic between the two countries had been interrupted for six hours and that Libyan air controllers had cited security reasons.
Some Tripoli residents, tired of daily fighting disrupting power and food supplies, hope NATO will intervene in Libya as it did in 2011 when the alliance sent jets to bomb Gaddafi forces in support of the uprising that toppled him.
On Sunday, the U.N. Mission in Libya said it "deeply regrets that there was no response to the repeated international appeals and its own efforts for an immediate ceasefire".
The new U.N. special envoy, Bernardino Leon, who is due to start his job on Sept. 1, said he might travel to Tripoli as early as this week. (Reporting by Feras Bosalum, Ahmed Elumami, Heba al-Shibani, Ulf Laessing, Asma Alsharif and Arshad Mohammed; Editing by Mark Heinrich)
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