By Sue Pleming

WASHINGTON, April 14 (Reuters) - The United States urged Chad, Sudan and other African countries on Friday to try to halt the violence in the region, saying the closure of borders with Sudan only exacerbated tensions.

"Things have heated up to a point where we have seen violence get out of hand, and it is incumbent upon all parties to do what they can to provide security to reduce those levels of violence, as well as to make sure that all international obligations that they have are lived up to," said State Department spokesman Sean McCormack.

Chad's president Idriss Deby has broken diplomatic ties with neighboring Sudan, which he accused of trying to topple him, and warned his country could stop sheltering thousands of Sudanese refugees who have fled the political and ethnic conflict in the western Darfur region.

Sudan has denied any responsibility for attacks on Chad, including a surprise raid on the capital on Thursday.

Washington had "grave concerns" about Chad sealing off its borders with Sudan and the impact on refugees as well as access to camps on both sides of the border, McCormack said.

"So we call upon the government of Chad to uphold its responsibilities, as outlined by the U.N. and international obligations, to provide protection for these refugees as well as to provide international access to the refugees," he said.

The Central African Republic also closed its border with Sudan to protest what it called Khartoum's "aggression" following rebel attacks on Chad.

The United States and other nations are worried about tensions between Chad and Sudan, particularly in Darfur where killings and other atrocities have raged for three years.

Deby assailed the international community for being too slow to react to the escalating border war and said if the Darfur crisis was not resolved by June, his country would no longer shelter more than 200,000 Sudanese refugees.

McCormack said kicking out the refugees would be "unacceptable, adding: "The government of Chad has certain responsibilities."

U.S. Deputy Secretary of State Robert Zoellick met African Union Commission Chairman Alpha Oumar Konare of Mali to discuss Sudan and urged him to deliver a strong message to Khartoum to accept an offer for U.N. peacekeepers to go to Darfur.

"It's important that the government of Sudan hear this message as strongly as possible, as often as possible, from others in the region," said McCormack"

The United States is pressing Khartoum to allow an AU mission in Darfur to come under the auspices of the United Nations, but Sudan has so far rejected the idea.