Tony Blair has called for an immediate ceasefire and political resolution to the conflict in Sudan's Darfur region.
Many thousands of displaced people are in need of relief supplies
He said continued attacks by the government of Sudan and rebel movements were prolonging the "terrible crisis".
He also hinted at sanctions if rapid progress was not made, warning: "We will consider alternative approaches with international partners."
But the Aegis Trust, which campaigns to stop genocide worldwide, said the prime minister was "wasting his breath".
The trust said calls for an immediate ceasefire, political process and effective peacekeeping force had been made for more than three years.
Chief Executive James Smith said: "What's needed now goes well beyond mere sanctions against the architects of ethnic cleansing in Darfur, though it's a disgrace that only now are these being hinted at. "
The trust is calling for measures including a strong UN peacekeeping force in neighbouring Chad and a UN-authorised non-fly zone over Darfur.
With much of Darfur inaccessible to aid workers and researchers, calculating how many deaths there have been in the past three years is impossible.
But research published in September in the journal Science estimated at least 200,000 more people than would otherwise have been expected had died since 2003.
That figure includes both those who have died as a result of violence and those dying as a result of starvation or disease in refugee camps. An estimated two million people have been driven from their homes
Sudan's government and the pro-government Arab militias are accused of war crimes against the region's black African population, although the UN has stopped short of calling the crisis genocide.
The government admits mobilising "self-defence militias", but denies links to Arab Janjaweed militia and says the problems have been exaggerated.
African Union (AU) peacekeepers are in Darfur until June 2007, after Sudan rejected plans for it to hand over to a larger, stronger UN mission.
On Sunday, which is International Human Rights Day, campaigners from around the world will stage a day of demonstrations calling for peace in Darfur.
Mr Blair said in his statement: "The terrible suffering of the Darfur people has gone on for far too long.
"The situation there is totally unacceptable. Two million people remain in camps and as many as four million are in need of food aid."
Mr Blair said "appalling human rights violations" were continuing, with many women and young people suffering rape and sexual violence on a regular basis.
He said the UK would continue to support recent agreements reached in Addis Ababa and Abuja on the deployment of peacekeepers.
"But both the government and the rebel movements should be clear that they will be judged on the basis of actions, not just words," he said.
"They must move forward quickly to implement an immediate and strengthened ceasefire, commit to a political process, and agree an effective peacekeeping force for Darfur."