A senior US envoy has said Washington is "very concerned" about a "build-up of military forces" in the war-torn Darfur region of Sudan.
AU forces in Darfur are stretched beyond capacity
US Assistant Secretary of State Jendayi Frazer has gone to Khartoum to try to persuade the ruling party to accept a United Nations peacekeeping force.
Washington has said urgent action must be taken to stop a "genocide".
The African Union has agreed to the UN taking over from its own peacekeepers, but Khartoum rejects having a UN force.
Ms Frazer said before leaving Washington that she would deliver a letter to Sudanese President Omar al-Bashir on her visit this weekend.
She said AU troops were stretched to "breaking point" and could not keep the peace in an area the size of France.
"Darfur is on the verge of a dangerous downward spiral," Ms Frazer said.
She stressed the US was not about to "fight its way in" and that any international peacekeeping force would need the backing of Sudan's government.
But she insisted that "foot-dragging at the UN" must not be allowed.
She also rejected a Sudanese proposal to send more government troops to Darfur.
"Those forces are not considered neutral and so we don't feel that the people of Darfur will get any comfort," said Ms Frazer.
The US and the UK had circulated the draft, calling for 17,000 well-equipped peacekeepers, at the Security Council.
However, Sudanese National Congress Party chairman Ghazi Salah Eldin Atabani said the plan would "impose complete tutelage" on Sudan.
"Any state that sponsors this draft resolution will be regarded as assuming a hostile attitude against the Sudan," he said.
On Sunday, two AU peacekeepers were shot and killed in Darfur, the AU says.
On Wednesday, the International Rescue Committee (IRC) said that incidents of rape near camps for those displaced by the fighting were becoming more common.
It is yet further evidence, relief workers say, that security is worsening in one of the most troubled regions of the world.
There has also been an escalation of attacks against humanitarian aid workers, some of whom have had to cease operations.
A peace deal was signed last May between the government of Sudan and a faction of the Sudan Liberation Army rebels, yet the bloodshed continues to force tens of thousands of people to seek refuge in camps.
Some two million people have fled their homes in the conflict, which began in February 2003 when Darfur rebels took up arms, accusing the government of ignoring the region.