Libyan government forces bombed large fuel storage tanks in the contested western city of Misrata, destroying the tanks and sparking a huge fire, rebels said on Saturday.
The bombardment came as artillery rounds fired by forces loyal to Libya's Muammar Gaddafi fell in Tunisia in an escalation of fighting near the border between Libyan soldiers and anti-Gaddafi rebels.
Misrata is the last remaining city in the west under rebel control. The port city has been under siege for more than two months and has witnessed some of the war's fiercest fighting between loyalists and rebels.
Rebels gave varying accounts of the Misrata bombardment but said the overnight attack, which hit fuel used for export as well as domestic consumption, came as a blow to their ability to withstand the siege.
"Four (fuel) tanks were totally destroyed and a huge fire erupted which spread now to the other four. We cannot extinguish it because we do not have the right tools," rebel spokesman Ahmed Hassan told Reuters.
"Now the city will face a major problem. Those were the only sources of fuel for the city. These tanks could have kept the city for three months with enough fuel," he said by telephone.
Hassan said government forces used small planes normally used to spray pesticides for the overnight attack in Qasr Ahmed. He later told Al-Jazeera television that three helicopters bearing Red Crescent insignia conducted the attack.
Another rebel spokesman, who gave his name as Abdelsalam, said a government helicopter conducted a reconnaissance mission over the port and two hours later at around midnight local time government forces fired rockets that hit three fuel tanks belonging to the Brega Oil Company.
Footage of the incident posted on YouTube by Libyan students in Misrata showed firefighters turning water hoses on a raging fire in a vain attempt to extinguish it.
Rebels notified NATO about the planes before the attack but there was no response, Hassan said. Government forces last month flew at least one helicopter reconnaissance mission over Misrata, according to rebels.
Fighting has intensified in Libya's Western Mountains region as Gaddafi loyalists and rebels, backed by NATO bombing, reached stalemate on other fronts in the civil war.
Government forces surrounding the rebel-held town of Zintan fired 300 rockets into the town on Saturday, said rebel spokesman Abdulrahman al-Zintani, who gave no details of casualties in Zintan, which is largely empty of civilians.
"NATO aircraft can be heard but there have been no air strikes," al-Zintani told Reuters.
In the Tunisian frontier town of Dehiba, schools were evacuated and residents scurried for safety as close to 100 mortars and missiles fell.
The crackle of small arms fire as well as larger weapons could also be heard about 4 km (2.5 miles) inside Libya, a Reuters witness on the border said.
"We are very afraid. The missiles are falling right around us, we don't know what to do," said Tunisian Mohammed Naguez, a resident of Dehiba. "Our children are afraid. The Tunisian authorities have to stop this."
The town has been hit repeatedly by stray shells in recent weeks as the Libyan rivals fight for control of a nearby border crossing but the government on Saturday denied it targeted Tunisian soil deliberately.
"We said this (shelling) was an error and we have apologised that this took place and have asked the military forces to ensure this doesn't happen again," Libyan Prime Minister Al-Baghdadi Ali al-Mahmoudi told a news conference in Tripoli.
The battle is for control of the Dehiba-Wazzin border crossing, which gives the rebels a road from the outside world into strongholds in the Western Mountains region where they are fighting to end Gaddafi's rule of more than four decades.
Although the rebels hold the Dehiba-Wazzin border point, Gaddafi's forces are in charge of a far bigger one to the north.
Most of the people in the Western Mountains belong to the Berber ethnic group and are distinct from other Libyans.
They rose up two months ago and say towns such as Zintan and Yafran are under repeated bombardment from Gaddafi's forces, running short of food, water and medicine.
Last week, fighting at the Tunisian border crossed into Dehiba itself, drawing furious protests to Libya from Tunisia's.