By Evelyn Leopold

UNITED NATIONS, March 6 (Reuters) - U.N. Security Council members voiced frustration on Tuesday that Sudan's President Omar Hassan al-Bashir had still not answered a U.N. letter sent six weeks ago on beefing up African peacekeepers in Darfur.

At issue is an interim force to bolster 7,000 African Union troops with some 3,000 personnel, mainly engineers, logistics and medical units as well as helicopter pilots. This group would plan for a far larger African Union-U.N. force.

Although agreed in principle, U.N. Secretary-General Ban Ki-moon wrote al-Bashir a letter six weeks ago to finalize arrangements and was promised an immediate reply.

"There is a lot of frustration among council members," South Africa's U.N. Ambassador Dumisani Kumalo, this month's council president, told reporters after a closed meeting.

The Africa Union and the United Nations wrote to Sudan on Tuesday, saying they had decided the final form of the peacekeeping force should include 19,000 to 20,000 troops and nearly 6,000 police in Darfur, said Hedi Annabi, an assistant secretary-general for peacekeeping. Sudan has in the past raised objections to such high numbers.

France's U.N. ambassador, Jean-Marc de La Sabliere, warned that the 15-nation council could consider sanctions if Sudan did not respond. The European Union has already urged more sanctions but Russia and China remain opposed.

"I am disappointed that we have not yet received the letter," de la Sabliere said. "If it does not come, then we'll have to see what to do and there are some delegations on the council thinking about taking measures."

POLITICAL PROCESS

The five permanent council members with veto power -- the United States, France, Britain, Russia and China -- held a private meeting called by Britain to discuss how to spur Khartoum into action.

"Security for people in Darfur is essential, human access is crucial and a political process to permit all that to happen with a cease-fire is what this is all about," Britain's U.N. ambassador, Emyr Jones Parry, told reporters.

"If that doesn't happen very soon, then clearly we are going to have to think about what we should do," he said.

The Darfur conflict, pitting government troops and allied Arab militia against splintered rebel factions, has killed tens of thousands of people and displaced millions since it broke out in 2003. Some 2.3 million people live in arid camps, and women are raped when they leave to collect firewood.

Ban has not yet appointed a special U.N. representative for Sudan. Swedish diplomat Jan Eliasson is in charge of a peace process between rebels and the government, but he has made clear he is not negotiating on peacekeeping troops.

Eliasson, who briefed the council on his recent negotiations on Darfur, said the Sudanese government at times was still backing militias, such as the Janjaweed, accused of rape, pillaging and murder. But he said it had stopped aerial bombardments as of Feb. 11 in north Darfur, were violence has decreased.

He said the various rebel movements, especially in south Darfur, were trying to consolidate their positions while fighting increased between tribes and clans that had little to do with government action.

Khartoum and one rebel group signed a peace agreement in May. Eliasson is trying to engage the other rebels groups to sign on. He said Sudanese officials indicated their readiness to consider amendments, but not to renegotiate the whole deal.

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