The Liberal Democrats have called for Britain to lead an EU military force dealing with the humanitarian crisis in Sudan.

The move came after the prime minister refused to rule out an armed intervention in the Darfur region where rampaging militias have forced thousands of refugees from their homes.

However Tony Blair said nothing had yet been decided. "We have a moral responsibility to deal with this and to deal with it by any means that we can," he said on Thursday.

Lib Dem spokesman Sir Menzies Campbell wrote to foreign secretary Jack Straw on Friday urging him to push for an EU-led force to take action to protect refugee camps and ease the passage of aid.

But such a move could be resisted by the Sudanese government in Khartoum which is already facing huge international pressure to crack down on the Islamic militia which it had been supporting to crush a civil war.

Sir Menzies said this diplomatic work was not working.

"It is becoming increasingly clear that food aid and pressure on the Sudanese government alone will not be sufficient to stem the impending disaster in Darfur," he wrote to Straw.

"An EU military force operating under a UN mandate looks like the only answer if we are to prevent a disaster on the scale of Rwanda a decade ago.

"With US and British forces stretched to breaking point, countries such as France and Germany have the opportunity to make a significant contribution. But Britain too must also contribute to any such force so far as it can.

"This has now become a matter of critical urgency. The Sudanese government must accept external military forces in Darfur expressly for the protection of aid relief and refugees. I hope that you will agree."

EU move

Straw is set to travel to Sudan next month and will discuss the crisis with fellow EU foreign ministers on Monday.

The Council of Ministers has said it "strongly supports" the role of the African Union in seeking to solve the crisis.

Earlier international development secretary Hilary Benn said Britain was doing everything it could, and was a key aid donor.

"We do have a moral duty to act," he told the BBC.

"The UK has done as much, if not more, than any other nation in the world."

However the Cabinet minister accepted that the response in Khartoum is still falling short of expectations and forcing the United Nations to get involved.

"They made it very difficult for the international community to get into Darfur," Benn said.

"Those things have now begun to change. Why? Because the international community has put pressure on the government of Sudan.

"We have seen progress on humanitarian aid. We have not yet seen enough progress on action to stop the militias attacking people.

"That is why we are going to the UN security council."

Meanwhile, the US is giving its backing to a UN resolution that threatens Sudan with sanctions if the crisis isn't eased.

And the US congress has passed a resolution condemning "genocide" in the region and seeking authorisation for a multinational force to protect displaced civilians and aid workers.