Western officials are scrambling to get authorisation for Libya airstrikes in the coming days before Islamic State captures the strategically important town of Ajdabiya, gateway to the country’s oil wealth.

Fierce fighting is raging in the town, which sits on a rocky plateau dominating the eastern oil ports. Capture will give Isis command of the Sirte basin, home to Libya’s largest collection of oilfields.

British, American and French jets are on standby for strikes from bases across the Mediterranean, with drones and reconnaissance planes already in the air. US special forces are in the Libyan desert, with one unit inadvertently photographed at the western Wattiya airbase last week.

But strikes will not be launched until Libya issues a formal invitation, which military planners hope will be made soon, after the United Nations last week unveiled what it hopes will be a new unified government for the chaotic country.

Western officials fear that, without Libya strikes, success in the intensified bombing of Isis in Iraq and Syria will be undone as it builds strength in north Africa. Isis is already reconfiguring globally, with volunteers from Sudan, Syria, Tunisia and Yemen moving to Libya, both by ship and along the “pipeline”, a smuggling network leading over the country’s long unguarded desert border.

The strategy of hitting Isis in Syria, Iraq and Libya at the same time was laid out on Friday night by Barack Obama, who said an American-led coalition would defeat Isis by “systematically squeezing them”.

On the ground, Ajdabiya is the focus. The town is Libya’s Aberdeen, transformed in the late 1970s from backwater to boom town by the oil industry. Sitting close to the coast, bisecting oil pipelines, its capture would leave Isis controlling a vast crescent, stretching 150 miles along the coast from its headquarters in Sirte and south through the oilfields.

“If Isis get Ajdabiya, then they have influence over the area,” says Libyan analyst Mohamed Eljarh. “Isis will keep playing on the weak spots in Libya to expand their territory.”

Intensive artillery fire and street fighting continues in the town, with the mayor reporting indiscriminate shelling and the hospital running out of supplies. The town’s strategic importance has seen it fought over and devastated since Roman times. Its capture by rebels in the 2011 Arab spring revolution, giving them control of the oil industry, sealed the fate of the dictator Muammar Gaddafi.

Four years ago, Nato bombing turned the tide in the fighting, and military planners think it can do the same now. A vast armada of aircraft is on standby. American F-15s, which bombed an al-Qaida gathering in the town in July, are at bases in Italy. RAF Typhoons and Tornados based in Cyprus, detailed for Syria bombing, can be switched south with the use of mid-air refuelling tankers. French reconnaissance planes are making passes over Isis bases and US special forces are criss-crossing the region.

America’s presence was highlighted last week with the inadvertent release of pictures showing a 20-man special forces team, with desert buggies and surveillance equipment, being turned away from Wattiya, 30 miles from the Isis base at al-Ajaylat, near Sabratha.

Libyan sources said the Americans had been in the country for weeks, using a range of special operations planes. Publicly available flight records show the special forces transport plane photographed at Wattiya, a blue-and-white Dornier C-146 Wolfhound, has made repeated journeys in and out of Libya, operating from an airbase at Pantelleria, a tiny Italian island between Sicily and Tunisia.

After last week’s abortive mission, records show it flying to Stuttgart, headquarters of the Pentagon’s Africom, from where Libya bombing missions will be coordinated. Those missions are on hold for political reasons, because until now Libya’s two warring governments, one in Tripoli, the other in Tobruk, have both refused to issue requests for military intervention.

Washington’s solution is to back the UN-brokered government of national accord, set up last week in the hope of uniting Libyans so their militias can concentrate on battling Isis. Diplomats expect the new administration to issue a formal invitation for airstrikes later this week. That is important for David Cameron, who wants to avoid another bruising vote on extending RAF strikes after the Commons decision on bombing Syria. A formal invitation will mean, as with Iraq, that Downing Street can order strikes without reference to parliament.

“Isis has been consolidating: they do now threaten the Sirte basin quite seriously,” said oil expert John Hamilton, director of London’s Cross Border Information. “If the west is determined, it will use the pretext [of an official invitation] and intervene.” The bombing, which is expected to start this week, will prioritise Ajdabiya, coordinating with militias to hit jihadi convoys outside the town.

Farther west, jets are expected to hammer Isis compounds at al-Ajaylat, which is a priority because of its role as training ground for terrorists sent abroad. Tunisia says the gunman who killed 38 tourists, 30 of them British, at Sousse in June, along with attackers who killed 23 at the Bardo museum in March, and 12 presidential guards in Tunis last month, were trained there.

Sirte will be a harder nut to crack. There, Isis is replicating its practice in Raqqa in Syria, using the civilian population as human shields.

Only ground troops are likely to decisively defeat Isis at Sirte and, while Britain and Italy are dispatching hundreds of soldiers to Libya to support the new government, only a handful of SAS are likely to see action against Isis.

Defeating the terror group will depend on the militias, and that depends on Libya’s existing factions falling into line behind a unity government which, lacking backing by the two parliaments, has sketchy constitutional authority.

“Bombing, in coordination with a strong Libyan army on the ground, it would work,” said Libyan commentator Nadia Ramadan. “Right now, it’s not clear if everyone will accept the unity government.”