The US says Sudan has so far failed to deliver on its pledge to help civilians in the troubled region of Darfur.
Janjaweed militiamen are accused of ethnic cleansing
Secretary of State Colin Powell made the statement a week after visiting Sudan and urging the government to disarm the Janjaweed Arab militia.
The UN Security Council has discussed a US plan to impose sanctions on the group, which is accused of atrocities.
African leaders have decided to send 300 troops to Darfur, where one million have been driven from their homes.
After last week's visits by both Mr Powell and UN Secretary-General Kofi Annan, the Sudanese government committed itself to the immediate disarmament of the dreaded Janjaweed.
But on Thursday Mr Powell said the authorities had not so far kept this promise.
"President Bush, the United States Congress, Secretary General Annan and the international community want more than promises," he said.
"We want to see dramatic improvements on the ground right now," he added.
The warning comes as the UN security council is debating a draft resolution on Darfur sponsored by the US.
Mr Powell said the text would include sanctions - including a travel and arms ban - on the Janjaweed and all those who aided and abetted them.
The Khartoum government has been accused of supporting the militia - a charge it denies.
A previous Security Council statement failed to criticise Khartoum directly, after resistance from Pakistan and China.
France has also said it does not support extending the sanctions to the Khartoum government itself.
The Security Council met in closed session for several hours on Wednesday and the draft resolution was again discussed on Thursday.
Council members also disagree over how long the Sudanese government should be given to resolve the situation itself.
The US draft resolution threatens to escalate the sanctions within 30 days if results are not evident.
But some countries say that Sudan should be allowed more time.
Earlier on Thursday, African leaders meeting in Addis Ababa decided that a protection force they agreed to send would defend Darfur civilians, as well as guard African Union ceasefire observers.
The Chairman of the African Union Commission, Alpha Oumar Konare, said the 300 troops would arrive in Sudan by the end of July.
The humanitarian situation is worsening
The BBC's Barnaby Phillips says the union is determined to be taken seriously as a body devoted to solving the continent's problems, but is severely hampered by a lack of resources.
African leaders say they hope richer countries will also do their bit to help.
At least 10,000 have been killed in what the UN calls "the world's worst humanitarian crisis".
Human rights activists say the Janjaweed are conducting a genocide against Darfur's black African population - although the UN and member states have refrained from using the word.
A rebellion broke out in Darfur early last year, when two groups took up arms, accusing the government of ignoring the region.
Those who have fled their homes say the Janjaweed ride on horses and camels into villages which have just been bombed by government aircraft, killing the men and raping the women.