UNITED NATIONS (Reuters) - 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 U.S. 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.

"We weren't given specifics on that," she said. "But we have seen, in the violence that is taking place in the South, a higher degree of sophistication and lethality of the weapons employed, and that's a source of concern."

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

"But I imagine that weapons are also coming from elsewhere and we would like a full accounting," Rice said, adding that it was a region with "porous borders" and that weapons were coming from "all directions."

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. It outlined the April elections and a southern Sudanese referendum on independence in 2011.

But delays in implementing the deal have fueled mistrust between the North and South. A law forced through last month by President Omar Hassan al-Bashir's dominant National Congress Party giving Sudan's feared intelligence services wider powers has further compounded matters.