(Updates with comments from Brown, Archbishop of Canterbury)

LONDON, Dec 31 (Reuters) - British Prime Minister Gordon Brown called on Wednesday for an "immediate and urgent" ceasefire in the Gaza Strip, where Israeli air strikes have killed hundreds of people.

"We have got to ... secure an immediate and urgent ceasefire," Brown said in his strongest statement yet on the conflict.

He said Palestinian President Mahmoud Abbas had put forward proposals that were being discussed by Arab ministers.

"I believe that this is the best opportunity for a ceasefire and for peace. I believe that the various forces involved should seize this opportunity now," he told reporters.

"It's vital that moderation must now prevail," he added.

Israel earlier rejected calls for an immediate ceasefire in the Gaza Strip, controlled by Islamist group Hamas. It stepped up preparations for a possible ground offensive after Hamas's long-range rockets hit another major population centre.

Brown said he had been assured by Israeli Prime Minister Ehud Olmert that Israel would allow humanitarian access to Gaza to get supplies to its inhabitants and to help with casualties.

"I have some indication that aid is now getting through," he said.

Britain pledged $10 million of emergency humanitarian aid on Wednesday for the Gaza Strip.

Israeli air strikes have killed 393 Palestinians, at least a quarter of whom, U.N. figures showed, were civilians. Three Israeli civilians and a soldier have been killed by rockets.

Israel says it is trying to halt Hamas rocket attacks.

In Gaza, basic food supplies were running low and power cuts affected much of the territory. Hospitals struggled to cope with casualties.

Britain's International Development Secretary Douglas Alexander said securing an aid route was urgent.

"The limited aid that is getting through cannot be distributed properly because of bombing from the air and rocket attacks launched from inside Gaza. The fighting must stop to enable humanitarian agencies to help people in desperate need," he said.

Archbishop of Canterbury Rowan Williams, spiritual head of the Anglican Church, urged political leaders to push for a truce in Gaza, suggesting that outgoing U.S. President George W. Bush and President-elect Barack Obama should combine in appealing for a halt to violence.

"We must unite in urging all those who have the power to halt this spiral of violence to do so," he said in a statement. (Reporting by Adrian Croft, Kate Kelland; editing by Philippa Fletcher)

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