The move marks the opening of a new front by the U.S. administration in the war against Islamic State, which, under added pressure in its Syria and Iraq strongholds, is increasingly resorting to planning attacks abroad.
Obama said the air strikes were undertaken to make sure that Libyan forces were able to finish the job of fighting the radical militant group and to increase stability there.
The United States, Europe and countries around the world "have a great interest in seeing stability in Libya because the absence of stability has helped to fuel some of the challenges that we’ve seen in terms of the migration crisis in Europe and some of the humanitarian tragedies that we’ve seen in the open seas between Libya and Europe," Obama told reporters.
Islamic State has struggled to win local support in Libya but has exploited the chaos that followed the ouster of dictator Muammar Gaddafi. Its presence in Sirte has been reduced to a few hundred fighters who once controlled what was Gaddafi's hometown.
A U.S. government source familiar with American intentions said the U.S. military's plan was to pulverize Islamic State militants through aerial bombardment to deny them the safe haven of Sirte, even though such a strategy risks dispersing the group's forces to neighbouring countries and beyond, where they may carry out attacks to show they are still "a force to be reckoned with."
The U.S. administration's tactics in Libya, two American officials said, were largely a product of the administration's use of manned and unmanned aircraft against Islamic State, and intended to avoid committing "any meaningful level of ground support."
There is little public or political support for sending U.S. ground troops to Libya, which remains deeply divided.
The United Nations-backed government, struggling to assert its authority over the fractured country, has hesitated to call for U.S. support until now for fear of a public backlash.
However, the U.S. officials said that if Islamic State’s growing external operations branch manages to mount a major terrorist attack in the West, "the administration may end up paying a higher price for this tactic than it would if it had decided to send the support that’s needed to encircle places like Sirte and Mosul (in Iraq) without turning to other groups."
U.S. warplanes so far have conducted strikes on seven targets in and around Sirte over the past two days, Pentagon spokesman Captain Jeff Davis said at a news briefing on Tuesday.
The targets included two tanks, an Islamic State fighting position, construction vehicles, military vehicles, a rocket launcher and an excavator.
The strikes already have helped Libyan forces gain ground against Islamic State, Davis said.
"That had really proved to be a menacing problem for the GNA," he said, referring to Libya's United Nations-backed Government of National Accord. "It was something they (Islamic State) had used repeatedly to beat back advances."
The strikes are being carried out by both crewed planes and drones, Davis said, declining to give further details on where the aircraft were based. Some Islamic State fighters have been killed in the strikes, he said, though he declined to give a number.