* Clashes in Benghazi, Tripoli worst since 2011
* International envoys call for immediate ceasefire
* New parliament due to begin in August (Adds attack on oil tanks)
By Feras Bosalum and Ayman al-Warfalli
BENGHAZI/TRIPOLI, July 27 (Reuters) - At least 36 people were killed in Libya's eastern city of Benghazi where Libyan Special Forces and Islamist militants clashed on Saturday night and Sunday morning, medical and security sources said.
The government said more than 150 people have died, many of them civilian, in the capital Tripoli and Benghazi in two weeks of fighting as clashes forced U.S. and foreign diplomats to pull out of the country.
In Tripoli, 23 people, all Egyptian workers, were killed when a rocket hit their home on Saturday during fighting between rival militias battling over the city's main airport, the Egyptian state news agency reported.
Since the clashes erupted a fortnight ago, 94 people have died in the capital, and more than 400 have been injured as militias exchanged rocket and artillery fire across southern Tripoli, the health ministry said.
Another 55 people, mostly civilians, have been killed in Benghazi since the clashes have intensified over the last week between regular forces and Islamist militants who are entrenched in the city.
"Most of the victims we have noticed are civilians as the fighters have their own hospitals on the battlefield," a Benghazi medical source told Reuters.
Fuel storage tanks that supply Tripoli were hit on Sunday by rockets igniting a huge fire near the international airport, the National oil corporation (NOC) said.
"It is a tank of 6 million litres of gasoline and it is close to others containing gas and diesel," NOC spokesman Mohamed al-Alharari said. "The firefighters are trying to counter the fire but if they cannot, a big disaster will happen" he added.
In the last two weeks, Libya has descended into its deadliest violence since the 2011 war that ousted Muammar Gaddafi, with the central government unable to impose order.
The United States, the United Nations and Turkey have pulled their diplomats out of the North African country.
The United States evacuated its embassy on Saturday, driving diplomats across the border into Tunisia under heavy military protection because of Tripoli clashes near the embassy compound.
A British embassy convoy was hit by gunfire during an attempted hijacking outside the capital Tripoli on the way to the Tunisian border, but no-one was injured in the incident, an embassy official said on Sunday.
"It was an attempted hijack as the convoy was on its way to the Tunisian border," the official said. "No one was injured but vehicles were damaged."
On Sunday, shelling continued in Tripoli around the international airport that is controlled by militias from the western city of Zintan. More Islamist-leaning rival brigades are trying to force them from the airport, which Zintanis have controlled since the fall of Tripoli.
But clashes were far heavier in Benghazi overnight, where regular army and air force units have joined with a renegade ex-army general who has launched a self-declared campaign to oust Islamist militants from the city.
A source from the Special forces fighting Islamist militants in Benghazi told Reuters clashes involved warplanes hitting militant positions belonging to Ansar al Sharia and another group in the city.
Libya's Western allies worry the OPEC country is becoming polarised between the two main factions of competing militia brigades and their political allies, whose battle is shaping the country's transition.
Special envoys for Libya from the Arab League, the United States and European countries expressed their concerns about the situation in Libya, saying it had reached a "critical stage" and called for an immediate ceasefire.
"The UN should play a leading role in reaching a ceasefire in conjunction with the Libyan government and other internal partners, with the full support of the international envoys," a statement issued after a meeting in Brussels said.
A new Libyan parliament was elected in June and Western governments hope warring parties may be able to reach a political agreement when the lawmakers meet in August for the first session.
But three years after Gaddafi's demise, Libya's transition to democracy has been delayed by political infighting and militia violence. Armed groups have also targeted the oil industry to pressure the state. (Additional reporting by Ahmed Elumami in Benghazi; Writing by Aziz El Yaakoubi; Editing by Patrick Markey, Angus MacSwan and David Evans)