Tripoli, Libya (CNN) -- British diplomats were on the ground in Libya meeting with key figures of the opposition, but no decision has been made by Western allies on whether to arm them, the UK Foreign Office said Sunday.

"What we are engaged in is protecting the civilian population in Libya, which we have done with a lot of success ... when people look at what we're doing in Libya they do have to look at what would be happening if we didn't do what we'd done over the last few weeks and it would have been a catastrophic situation," UK Foreign Secretary William Hague said.

A spokesperson for the office said the goal of the British diplomatic team is to build on the work of a previous team and to "establish further information about (the opposition), its aims, and more broadly, what is happening in Libya."

Meanwhile, the United States agreed to extend until Monday the use of its strike aircraft over Libya due to poor weather conditions over the past few days, NATO spokesman Oana Lungescu said.

"These aircraft will continue to conduct and support alliance air-to-ground missions throughout this weekend," he said.

The deadly battles in Libya forged ahead Sunday as pro-government forces shelled a medical clinic in the city of Misrata, killing one person and wounding 15 others, a hospital source said.

The source, a doctor who was not identified for security reasons, told CNN two people were injured by an initial mortar blast. The rest of the injured were wounded by a second mortar blast when they went to the scene of the first attack to help victims.

One of the injured is a 14-year-old child who suffered a fractured skull and is in a coma, the doctor said Sunday.

The clinic that was attacked had evacuated patients because of recent attacks, said another doctor at a Misrata hospital that received the patients. But it was being guarded by opposition "fighters and young people" who were injured.

Elsewhere in the city, the sound of heavy shelling was heard coming from the port area, a resident said.

He said troops supporting Libyan ruler Moammar Gadhafi surrounded the city, with snipers perched on rooftops of buildings.

Misrata's port is under the control of the rebels and is considered the main site of aid for the people in the embattled city. A bombing in the port area could prevent aid ships from reaching Misrata.

Sunday's events are the latest in the struggle between forces loyal to Gadhafi and opposition members seeking an end to his nearly 42-year rule.

Rebel fighters retreated east of the oil town al-Brega on Sunday. Fighters said there was some sort of ambush, they saw roadside mines and they needed to get more ammunition. They regrouped about 30 to 35 kilometers (18 1/2 to 22 miles) east of al-Brega.

NATO is leading international military operations in the country following a U.N. Security Council resolution authorizing force and the enforcement of a no-fly zone to protect civilians.

NATO airstrikes hit several rebel vehicles and killed at least 13 rebel fighters, spokesmen for the Libyan opposition said Saturday. Seven others were wounded.

"Based on the information we have, they (the opposition forces who were hit) heard the airstrikes and went ahead to see what the damage was, and that's when they got hit," rebel spokesman Shamsiddin Abdulmolah said. "They were told to stay back, but they jumped the gun."

NATO was investigating the incident, a spokeswoman said Saturday.

"NATO takes any reports of civilian casualties very seriously, but exact details are hard to verify, as we have no reliable sources on the ground," NATO's Lungescu said.

On Saturday, notions of a cease-fire in the country quickly faded as a battle for control continued in al-Brega. It was not immediately clear who had control of the coastal city, which has changed hands six times in as many weeks under the dramatically shifting circumstances of the Libyan war.

A former Obama national security adviser said Sunday that the international coalition in Libya cannot leave the country until Gadhafi leaves power.

"The problem is that while everybody wants to see Gadhafi leave, either be removed or leave on his own, that end state is not yet clear," Ret. Gen. Jim Jones said Sunday on CNN's "State of the Union." "Unfortunately most people want perfect clarity in a situation where clarity doesn't really exist yet."

Last week, Libyan opposition leader Mustafa Abdul Jalil laid out cease-fire conditions that included freedom of expression for the Libyan people and the removal of snipers, mercenaries and militias from western cities. Ultimately, he said, the opposition's goal remains regime change in Libya.

But government officials spurned the opposition cease-fire proposal.

Government spokesman Musa Ibrahim told reporters in Tripoli on Friday the offer included "silly conditions."

"They are asking us to withdraw from our own cities and open our cities to people, who are holding up arms, who are tribal, violent, no unified leadership, al Qaeda links, and no one knows who they are," he said. "If this is not mad, then I don't know what it is."

Rebel spokesman Abdul Hafiz Ghoga sought to clarify the opposition's position Saturday.

"There is no, and was no, negotiation on a cease-fire with Colonel Gadhafi's dictatorship," he said at a news conference.

He repeated the opposition demands that Gadhafi halt all military action, end the sieges laid on cities like Misrata and allow free speech and assembly.

Sources close to Gadhafi have told CNN that political solutions are still possible but that the Libyan leader would relinquish power only to others within his inner circle.

The rebels have been hampered by a lack of organization and training on heavy weaponry when confronting the better-trained, better-armed forces of Gadhafi, who is under investigation for alleged crimes against humanity by the International Criminal Court.

CNN's Ben Wedeman, Nic Robertson, Reza Sayah, Eve Bower and Yousuf Basil contributed to this report