Nato has decided to boost surveillance flights over Libya, a senior US official said as the alliance debates imposing a no-fly zone over the country.
US Ambassador to Nato Ivo Daalder says allies had agreed to increase AWACs flights from 10 to 24 hours a day. The expansion is part of contingency planning for possible military intervention in Libya beyond humanitarian efforts.
The decision came as the alliance's governing board met to discuss what Nato could bring to Libya.
He said other ideas being considered were retasking Nato vessels in the Mediterranean Sea along with nearby air assets to deal with humanitarian aid and establishing a command and control structure to co-ordinate relief efforts.
Earlier President Barack Obama warned Libya's leaders that the US and Nato were still considering military options in response to what he called "unacceptable" violence perpetrated by supporters of Muammar Gaddafi.
"I want to send a very clear message to those who are around Colonel Gaddafi. It is their choice to make how they operate moving forward. And they will be held accountable for whatever violence continues to take place," Mr Obama said.
Libyan warplanes launched multiple air strikes on opposition fighters in the second day of a government crackdown to thwart rebels advancing on Gaddafi's stronghold in Tripoli.
White House spokesman Jay Carney said a military response was no more likely today than it was before the surge in violence. He said the US and its partners were considering a wide variety of military actions, including a no-fly zone, but said deploying ground troops was "not top of the list at this point".
Mr Carney said the US was also considering providing weapons to rebel forces, though he cautioned that there were still many unanswered questions about what groups comprise those forces. He said the US was using diplomatic channels, as well as contacts in the business community and non-governmental organisations, to gather information about the opposition.
Mr Obama said he had authorised 15 million dollars in humanitarian aid to help international and non-governmental organisations assist and evacuate people fleeing the violence. More than 200,000 people have fled the country, most of them foreign workers, creating a humanitarian crisis across the border with Tunisia - another North African country in turmoil after an uprising in January that ousted its long-time leader.
Hundreds if not thousands of people have died since Libya's uprising began, although tight restrictions on media make it nearly impossible to get an accurate tally.
The US and United Nations have imposed sanctions on Gaddafi's regime, and US military forces have moved closer to Libya's shores to back up demands that Gaddafi step down.
Britain and France are drafting a UN resolution that would establish a no-fly zone over Libya.
A British diplomat at the UN has stressed that the resolution is being prepared as a contingency in case it is needed, but no decision has been made to introduce it at the UN Security Council.
Libya's UN ambassador and his deputy have urged the council to impose a no-fly zone to prevent Gaddafi's forces from bombing civilians. Rebel fighters have also urged the establishment of a no-fly zone, saying they can take on Gaddafi's elite ground forces, but are outgunned if he uses his air power.