Plans for a possible no-fly zone over Libya are moving closer in response to air strikes against rebel-held areas by Libyan leader Col Muammar Gaddafi.
The UK and France are drafting a UN resolution for an air exclusion zone, which is due to be debated by Nato defence ministers on Thursday.
Gulf Arab states have already backed the idea.
But the BBC's Barbara Plett at the UN says the international community is divided over the issue.
Libyan government forces are threatening the key oil port of Ras Lanuf, after pushing back a rebel advance along the north coast.
Using air strikes, helicopter gunships and heavy armaments, pro-Gaddafi forces retook the nearby town of Bin Jawad on Monday.
The prospect of a no-fly zone gathered pace after Gulf states supported the idea and also called for an urgent meeting of the Arab League.
They have condemned the use of violence against civilians by Libyan government forces.
UK Foreign Secretary William Hague said Britain was working "with partners on a contingency basis on elements of a resolution on a no-fly zone".
But he said Arab and African support would be crucial, and there would need to be a clear trigger - most likely a sharp worsening of conditions for civilians.
Mr Hague also said the move needed the legal backing of a UN resolution - an element also required by the US and Nato.
Nato Secretary General Anders Fogh Rasmussen also stressed the need for UN authorisation for any no-fly zone.
"I can't imagine the international community and the United Nations would stand idly by if Gaddafi and his regime continue to attack their own people," he said.
The US said on Monday it was still considering a possible military response to the situation in Libya, along with its allies.
White House spokesman Jay Carney said arming rebel groups was "one of the range of options that is being considered", but stressed that Washington was still evaluating those groups opposed to Col Gaddafi.
US Defence Secretary Robert Gates said the Pentagon had prepared a number of military options for President Obama.
"I think at this point there is a sense that any action should be the result of an international sanction before anything is done," he added.
Correspondents say rebels fought off a strong counter-offensive as Gaddafi forces attacked towns near Tripoli and also in the east on Monday.
As the fighting intensifies, fears of a humanitarian crisis along Libya's borders are growing.
UN emergency relief co-ordinator Valerie Amos said up to one million foreign workers - who have either just left Libya or are still trapped there by the fighting - will need emergency aid in the coming weeks.
She said that $160m (£98m) was needed for camp management, food, nutrition, healthcare, water, sanitation and hygiene.
Unconfirmed reports quoting a rebel source said Col Gaddafi had sent a message to the rebels, offering to step down on condition he and his family were allowed to leave Libya safely with their wealth intact.
The rebel leadership in Benghazi - which calls itself the Transitional National Council - is said to have rejected the offer because it would have amounted to an "honourable" exit for the Libyan leader.
The BBC's Jon Leyne, who is in Libya's second city of Benghazi, says any suggestion that Col Gaddafi would ask the rebels' permission to leave the country should be treated with scepticism.
Our correspondent says the Libyan leader has always insisted he holds no official position, so there is nothing for him to relinquish.
The reports were made by al-Jazeera and two Arab newspapers.
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