US Secretary of State Condoleezza Rice has accused Sudan of failing in its responsibility to protect its own citizens in the western Darfur region.
The AU's mandate has been extended for three months
Speaking at a meeting organised on the sidelines of the UN General Assembly, Ms Rice said time was running out.
She hinted that, should Sudan continue to refuse access to UN peacekeepers, "other measures" were available.
Sudan's government vehemently opposes such a force, arguing that there is a hidden agenda to weaken the country.
"The violence in Darfur is not subsiding, it is getting worse," Ms Rice said.
"If the notion of our responsibility to protect the weakest and most powerless among us is ever to be more than an empty promise, then we must take action to save lives."
The US administration has described the crisis in Darfur, in which 200,000 people may have died and more than two million people have been displaced, as genocide.
Earlier this week, US President George W Bush named an envoy whose role will be to pressure Khartoum to allow a UN peacekeeping force into the country.
The African Union, whose 7,000 soldiers correspondents say are ill-equipped and underfunded, voted this week to extend its mandate, due to run out at the end of the September, for a further three months.
Foreign ministers of the Security Council and representatives of ten other nations agreed to step up pressure on Sudan to accept a UN force by January.
However, Ms Rice did not rule out other options.
"No one intends to appease the government of Sudan. We intend to act," she said, adding, "There are measures at the disposal of the international community should we not be able to get the agreement of Sudan."
Earlier, addressing the General Assembly, UK Foreign Secretary Margaret Beckett said that the three-month extension of the AU force could only be a temporary reprieve.
She said action was needed immediately on the political and humanitarian front, where aid groups were having increasing difficulty in delivering supplies.
Mrs Beckett also called for the active engagement and support of Asian and African countries, particularly Muslim nations.