U.S. President Barack Obama says he will keep a "full range of options" available as he deals with the crisis in Libya, where rebels continue to battle with forces loyal to leader Moammar Gadhafi.

"I don't want us to be hamstrung," Obama said Thursday during a White House news conference, saying he wants to be able to respond quickly to a humanitarian crisis or violence against civilians. As part of that preparation, 400 U.S. Marines arrived at an American naval base in Crete.

Obama reiterated the U.S. stance that Gadhafi has lost his legitimacy to rule and should step down.

Obama also announced that U.S. military aircraft would play a humanitarian role by flying Egyptians who had fled Libya home to Egypt from makeshift camps in Tunisia.

The president stressed that though he wants the U.S. to be able respond quickly to any circumstance, he'd prefer the world was on-side.

"I want us to make our decisions of what's going to be best for the Libyan people in consultation with the international community," he said.

Obama also warned people close to Gadhafi that they will be held responsible for attacks against civilians.

"They should know history is moving against Col. Gadhafi," he said.

War crimes probe against Gadhafi opened

The warning about war crimes Thursday echoed similar comments by the International Criminal Court's chief prosecutor, who opened the court's investigation into alleged crimes against humanity involving Gadhafi, some of his sons, and his inner circle.

"There will be no impunity in Libya," Luis Moreno-Ocampo told reporters in The Hague on Thursday.

Moreno-Ocampo said alleged incidents at various locations throughout the country starting on Feb. 15 will be investigated.

"No one has authority to attack and massacre civilians," Moreno-Ocampo said, warning he will also investigate opposition groups if necessary.

Moreno-Ocampo said the court will investigate those most responsible for the most serious incidents, including people who lead military or security groups.

"If forces under their command and control commit crimes, they could be criminally responsible," he said.

Rebels emboldened by battle

Meanwhile, in Libya, the government tried to seize the strategic oil town of Brega, launching air strikes for the second straight day.

Forces loyal to Gadhafi battled rebels armed with machine guns and rocket launchers Thursday, a day after the rebels managed to foil an attempt by Gadhafi's forces to retake the port in rebel-held east Libya.

Government warplanes launched a new air strike on the town Thursday morning, according to witnesses. It was not clear what they targeted, but it was likely an airstrip that belongs to the huge oil complex on the Mediterranean coast. There were no reports of casualties.

A rebel holds his ears as a bomb launched by a Libyan air force jet loyal to leader Moammar Gadhafi explodes in the desert near Brega Wednesday. (Joel Silva/Folhapress/Reuters)

"We are in a position to control the area and we are deploying our forces," a rebel army officer in Brega told The Associated Press, speaking on condition of anonymity because he was not authorized to talk to the media.

Wednesday's attack on Brega, about 740 kilometres east of Gadhafi's stronghold in the capital of Tripoli, marked the regime's first counteroffensive on the eastern half of the country, which fell quickly under opposition control after the revolt began on Feb. 15.

It was the latest reminder of the deep difficulties the regime's forces — an array of militiamen, mercenaries and military units — have faced in trying to roll back the uprising against 41 years of Gadhafi rule.

CBC's Carolyn Dunn said Thursday from the nearby town Ajdabyia that the rebels were buoyed by Wednesday's victory.

"What we have here is a people that have been mobilized by the attacks," she said, shortly after leaving a funeral for rebels killed in Wednesday's fighting. "They managed to push back on the government forces that came through, and that has basically emboldened them and they stand ready to fight again."

CBC's Tom Parry talked to one fighter in Ajdabyia, Madhid Suliman Ali, who boldly predicted a quick rout of Gadhafi's forces.

"Trust me, just give us 15 days, we're going to go to Tripoli," he said. "Fifteen days, Tripoli will be in our hands. Because God he is with us."

Opposition leaders are pleading for foreign powers to launch airstrikes to help them oust Gadhafi as the United States moves forces closer to Libyan shores to put military muscle behind Washington's calls for Gadhafi to give up power immediately.

"The leadership is messaging very clearly that they would like targeted air strikes on Gadhafi's troops, on his communications, and on certain buildings, and then they would like a no-fly zone imposed," Dunn said. "But… when they say that they always add at the end of it: 'But no foreign boots on the ground.' "

There is no international consensus on a no-fly zone. Britain and France have said if civilian deaths continue a no-fly zone will be needed, while Germany and Russia have opposed the idea. The U.S. said Wednesday that it was "a long way" from making a decision and warned that starting a no-fly zone would require an initial airstrike on Libya.

A rebel soldier gestures atop a car as he heads to Brega, Libya, on Wednesday. (Goran Tomasevic/Reuters)

Anti-government forces took to the streets more than two weeks ago to demand Gadhafi's ouster. The longtime leader has resisted calls to step aside and has repeatedly blamed al-Qaeda for the unrest in the north African nation.

Chavez offers to mediate

Venezuelan President Hugo Chavez has offered to be a mediator between the Libyan government and the rebels — an offer that has been rejected out of hand by anti-Gadhafi forces. A Venezuelan official said that the Arab League is interested in the proposal and that Gadhafi has accepted.

There are no details about how Chavez might broker a peace agreement, but Chavez and Gadhafi have a long-standing friendship.

The United Nations estimates at least 1,000 people have been killed since the unrest began, but other organizations say the death toll could be closer to 2,000.

The UN, EU, Britain and Canada have hit Libya and its leadership with a series of economic, travel and arms sanctions.

A Canadian military Hercules transport arrived in Malta Thursday with another 31 evacuees fleeing Libya — 14 of them Canadians.

About 250 to 260 Canadians have been taken out of Libya since the crisis began, according to Foreign Affairs Minister Lawrence Cannon. About 100 to 150 Canadians remain in the country, he said. Many are in locations that are more difficult to reach, and others don't want to leave.

With files from The Associated Press and The Canadian Press