United Nations - The United States is worried about the flow of arms into semi-autonomous southern Sudan, some of it heavy weapons, ahead of a nationwide April election, the US envoy to the United Nations said on Tuesday.

"We heard today from the U.N. that it is not just small arms but some heavier munitions that seem to be flowing in," U.S. Ambassador Susan Rice told reporters after a U.N. Security Council meeting on Sudan.

She added that Washington believed some of the weapons were coming from northern Sudan.

Human Rights Watch warned on Sunday that repression of political opponents in both Sudan's North and semi-autonomous South was undermining the prospects for Sudan's first democratic elections in 24 years, scheduled for April.

After decades of north-south civil war, a 2005 peace deal shared power and wealth and enshrined democratic reform in Africa's largest country.