The European Union calls on South Sudan and Sudan to settle outstanding issues without delay or condition, a statement read.
South Sudan last weekend marked its first anniversary of independence from Sudan. Independence was gained as part of a 2005 peace agreement that ended Sudan's bloody civil war. Unsettled issues, like oil distribution and borders, have threatened to derail the fragile peace, however.
Catherine Ashton, the top foreign policy official for the European Commission, praised South Sudan on its first year of independence but expressed concern over lingering security issues.
"Lasting peace and development in South Sudan will only be possible once South Sudan and Sudan live side by side as good neighbors respecting each other's sovereignty," she said in a statement.
Both sides were pushed to the brink of war this year following disputes over oil fields situated along the ill-defined border separating the two countries.
U.S. Ambassador to the United Nations Susan Rice, who attended independence celebrations last year, said the government in Juba can rely on Washington as a steadfast partner. However, there are "crucial unresolved issues" remaining for the world's newest nation.
Rice said last month following briefings at the U.N. Security Council that South Sudan and Sudan "remain closely poised, in close proximity, and seemingly on a hair trigger."