The top United States military officer says air strikes have hobbled Libyan forces but admitted the conflict was moving into stalemate.
Rebels have welcomed American plans to deploy unmanned aircraft, typically operated remotely from the United States.
But it emerged bad weather had forced the first two drones sent to Libya to turn back.
US Defence Secretary Robert Gates said the Predator drones offer more precise targeting, because their low-flying capability allows for better visibility, especially on "targets that have started to dig themselves into defensive positions."
Admiral Mike Mullen, chairman of the US military's joint chiefs of staff, said in Baghdad that the situation in Libya was certainly moving towards a stalemate.
Western forces began air raids on Libya on 19 March to enforce a United Nations-sanctioned no-fly zone and to protect civilians from attacks by government troops.
NATO subsequently agreed to take over the responsibility of enforcing the no-fly zone.
Rebels 'capture Libyan border crossing'
Rebel forces are reported to have taken control of a border post on the frontier with Tunisia after a brief clash with forces loyal to Colonel Muammar Gaddafi.
It is the first advance by the rebels for some time and happened in an area of the country almost entirely controlled by forces loyal to the Libyan leader, the BBC reports.
Reports say the rebels attacked the Wazin crossing early in the morning, prompting dozens of soldiers, including officers, to flee into Tunisia and surrender to authorities there.
The rebels control much of eastern Libya, while fighting continues in the besieged western city of Misrata. Doctors say nine insurgents have been killed in the latest clashes.
Rebel fighters, outgunned by government forces, describe a bitter, block-by-block war of attrition amid shattered buildings where the enemy is sometimes only metres away.
Hundreds of people are believed to have died in Misrata during the siege.
UN Secretary General Ban Ki-moon has called on the Libyan authorities to end the fighting and to "stop killing people".
Mr Ban said the UN's priority was to bring about "a verifiable and effective ceasefire" to enable the expansion of humanitarian assistance and political dialogue.