The United States is expressing concern that recent military moves by the Sudanese government could upset efforts to end the nearly two-decade-long conflict between the Khartoum government and southern rebels. Peace talks between the two sides in Kenya resumed Thursday.

The Bush administration applauded the resumption of the peace talks in a suburb of Nairobi, but at the same time it voiced apprehension over attacks and a military buildup by Sudanese government forces that officials here say appear to violate the cease-fire deal the two sides concluded in mid-October.

Briefing reporters, State Department spokesman Richard Boucher said the United States is "particularly troubled" by the government attacks on rebels in the Western Upper Nile region of Southern Sudan. He said the United States reminds both parties that they made a commitment to desist from military activity in the October memorandum of understanding:

"We're closely monitoring reports of fighting instigated by the government of Sudan, and we and the rest of the international community fear that this violation of the spirit and the letter of the October agreement will lead to continued fighting," he said. "It's incumbent on both parties to abide by the October memorandum of understanding and to end the fighting. Both the government and the SPLM have told the United States that they would pursue that course. Now is the time for them to act."

The new round of talks, which began a week late due to haggling over the agenda, is the third in a series of negotiating rounds that began in July under the auspices of the East African grouping IGAD, the Intergovernmental Authority on Development.

The Bush administration has actively supported the peace efforts, sponsoring a series of policy workshops for government and rebel representatives in Washington in December, and sending the U-S special envoy for Sudan former Senator John Danforth to the region for talks earlier this month.

In addition, the U.S. Congress approved and President Bush signed into law in October a measure, the Sudan Peace Act, that threatens sanctions against Khartoum if it fails to negotiate in good faith but also authorizes $300 million in U.S. aid over three years to support the peace process.

The peace talks produced a major break-through in July, when the Muslim government in Khartoum and the mostly-Christian and animist southern rebels agreed in principle to hold a referendum on possible southern succession after a six-year transition period.

The current round is expected to last two weeks and will deal with key issues including power-sharing during the transitional period and distribution of Sudan's oil wealth.

The civil conflict in Sudan erupted in 1983 and has killed an estimated two million people in combat and war-related famine and uprooted millions more.