"If we are going to get humanitarian relief and allow 50,000 [displaced] people to return, we first have to get a sense of security back there and that means a much different arrangement than we've had to date," he said.
Sudanese government forces control the main town in Abyei after a round of heavy fighting ended on May 20, and UN peacekeepers are patrolling the streets.
A US-mediated 2005 peace deal that ended Sudan's 21-year civil war between north and south state that joint Sudanese force patrols should be in place in the Abyei region.
Under the deal, Abyei was accorded a special status and was to be governed by a joint administration.
Twin referendums were earmarked for 2011 to decide whether it remains part of the north or joins the south, and whether the south will secede.
|Parts of Abyei saw bitter fighting between |
government forces and SPLM loyalists
The SPLM is the main coalition partner of the National Congress in the unity government set up under the peace deal.
Williamson said the deployment of a more effective international peacekeeping mission should be "considered".
"There are other options that also have to be considered but the principle of security in Abyei is uncompromisable," he said.
Williamson said he would hold talks with the SPLM to discuss how Abyei would be resolved.
"We think there is a way forward, that there's no excuse to allow this failure to result in ending the chance of real peace in Sudan," he said.
Yaser Arman, the SPLM's deputy secretary-general, said US mediation was needed to help implement a protocol on Abyei.
"We in the SPLM see very clearly that the efforts of the United States and the efforts of the special envoy are needed, especially in the implementation of the Abyei Protocol," said Arman.
The National Congress says the south has sparked the fighting by unilaterally appointing its own governor for Abyei.
Deng Alor, Sudan's foreign minister, said peaceful conditions had to be created on the ground so that those displaced by recent fighting can return home.
"For the security situation to change, the South Sudan Armed Forces and any SPLA should also go away so that the people [can] go back," he said.
The SPLM said last week that it would boycott talks between the government and the US in response to the violence in Abyei.
The UN has warned that the conflict could endanger the three-year peace process and threaten a return to civil war.