FACTBOX: Key facts about the conflict in Darfur

(Reuters) - The United States tightened sanctions against Sudan on Tuesday in hopes of putting additional pressure on the government of Sudanese President Omar Hassan al-Bashir to end the bloodshed in Darfur.

New measures block the assets of Sudanese individuals, including two high-ranking government officials, for their alleged roles in fomenting violence in the Darfur region. Thirty-one companies owned or controlled by the Sudanese government were also barred from the U.S. financial system.

The United States and Britain have demanded Sudan accept a strong U.N. peacekeeping force in Darfur, where four years of conflict have killed at least 200,000 people.

Here are some facts about the conflict in the Darfur region.

* THE CONFLICT:

-- Rebels in Sudan's western region of Darfur rose up against the government in February 2003, saying Khartoum discriminated against non-Arab farmers there.

-- Khartoum mobilized proxy Arab militia to help quell the revolt. Some militiamen, known locally as Janjaweed, pillaged and burned villages, and killed civilians. The government has called the Janjaweed outlaws and denied supporting them.

-- Experts have estimated that at least 200,000 people have been killed and over 2 million driven from their homes in the region since early 2003, some crossing the border into Chad, exacerbating a refugee crisis there.

-- The United Nations calls Darfur one of the world's worst humanitarian crises. The United States says the violence in Darfur amounts to genocide.

* CEASEFIRES:

-- A ceasefire was agreed in Darfur in April 2004. The African Union sent around 7,000 peacekeepers with a mandate to monitor the peace and protect those displaced in the camps. The ceasefire has been violated frequently, with fighting blamed on government troops, rebels and Janjaweed militias.

-- A peace deal in May 2006 was signed by only one of three rebel negotiating factions. The agreement was almost immediately rejected by many in Darfur who said it did not go far enough in ensuring their security. A new rebel coalition has since formed and renewed hostilities with the government.

* PEACEKEEPING FORCE FOR DARFUR:

-- In August 2006, the U.N Security Council adopted a resolution for deployment of a "hybrid" U.N.-AU force of 22,500 in Darfur to replace and absorb the present African Union force, which has been unable to stem the violence.

-- It invited the consent of Sudan, which has so far refused.

-- However, Sudan has agreed to allow an operation involving technical U.N. support personnel to deploy to Darfur. The "heavy support package" includes some 3,500 U.N. military and police personnel.