TOBRUK, Libya — Moammar Gadhafi’s warplanes, artillery and mortar shells can control huge swaths of territory by day, including oil ports, rebel supply routes and even hostile towns. Rebels say anti-government forces can still return in darkness to take advantage of Gadhafi’s own thin supply lines and overstretched ground troops.

The eastern port city of Brega has gone back and forth with the setting of the sun in recent days and is key to the battle for Libya’s oil centers — so key that both sides claimed control of it nearly simultaneously on Monday. The regime offensive appears to be hampered by a lack of manpower: They can drive out rebels with barrages, but not necessarily hold the territory.

Rebels, on the other hand, didn’t dare come out in the open on Monday in Brega, with a spokesman saying they were taking cover instead in the industrial oil area where they believed Gadhafi forces wouldn’t fire.

Brega and the city of Ajdabiya about 35 miles away again came under government bombardment on Monday, freshly exposing their importance as key crossroads for rebel supply lines, a main weakness in the Libyan region that contains most of its oil wealth. To get ammunition, reinforcements and arms to the front, they must drive along open desert highways, exposed to airstrikes. Gadhafi warplanes struck at least three targets Monday morning in Ajdabiya, missing a weapons storage site but hitting rebel fighters at a checkpoint in an attempt to stop supplies, rebels said.

Oil installations — and the ports that allow Libyan crude exports — are just as key as supply lines, and so the government and rebels both went out of their way late Monday to claim victory in Brega at nearly the same time, with a state television reporter in the town going so far as to show the hour on his watch.

Production has been cut drastically since fighting began and new questions arose Monday about whether the OPEC member was still exporting crude at all. Marsa al-Harigah, the last major oil port firmly under rebel control, is not expecting another tanker for a month, said Rajab Sahnoun, a top executive with the Arabian Gulf Oil Co., and its two functioning storage tanks could be full soon, forcing a production shutdown.

The rebels have pleaded for the West to impose a no-fly zone. France and Britain stepped up calls Monday for other world powers to isolate Gadhafi, but other countries, including the United States, have been cautious about backing the rebels.

British Prime Minister David Cameron said NATO was drawing up contingency plans for a no-fly zone.

“Every day Gadhafi is brutalizing his own people. Time is of the essence,” Cameron told the parliament in London. “There should be no let up in the pressure we put on this regime.”

Meanwhile, fighting raged in Brega, said Abdul-Bari Zwei, a rebel spokesman. He said the rebels controlled the neighborhoods, but Gadhafi forces were pounding them with bombs from the air, land and sea. He said the rebels were hiding in parts of the industrial oil area, believing Gadhafi forces would hold fire there.

“They won’t fire at the fuel trucks, they (Gadhafi’s forces) need them,” said Zwei.

Libya’s east is home to roughly 70 to 75 percent of the country’s reserves — the largest in Africa — and Gadhafi has every reason to try to regain control of the region quickly.

Government troops have scored victories using overpowering bombardments with artillery, tanks, warplanes and warships. Such an assault drove rebel fighters out of the oil port of Ras Lanouf several days ago.

After fleeing the bombardment Sunday, the rebels then pushed back into Brega in the evening and claimed to have captured dozens of fighters from Gadhafi’s elite Khamis Brigade.

On Monday, about 2,000 rebel fighters — mainly members of a special commando unit that defected to the opposition — held Brega’s residential district, while pro-Gadhafi troops controlled the industrial oil facilities some distance away, said Zwei. Rebel fighters were searching the residential area for any remaining Gadhafi troops.

Libyan state TV showed images Monday from Brega’s port, claiming it was in government control and at peace. The announcer urged Russia, China and India to invest in Libya’s oil.

Western Libya remains Gadhafi’s stronghold, centered on Tripoli where his militiamen have crushed any attempts at an uprising. But since early on in the revolt, which began Feb. 15, several cities in the west fell into rebel hands. Regime forces on Friday took back the most crucial of those cities, Zawiya, which lies on the capital’s doorstep, after a reportedly bloody and destructive week-long siege.

On Monday, pro-Gadhafi forces launched an attempt to take another, nearby town, Zwara, 70 miles west of Tripoli, close to the Tunisian border.

Government troops surrounded the town of 45,000 and bombarded it with tanks and artillery for hours starting in the morning, several residents said.