(CNN) -- Thousands of people trying to return to South Sudan from Sudan have been stranded for months at the Kosti way station and are running out of "means of support," a United Nations official said Tuesday.
The concern for their plight comes amid reports of military attacks along Sudan's hotly contested border with South Sudan.
"As those awaiting repatriation from Kosti to South Sudan are in a vulnerable position and lack many basic services including adequate shelter, vital and quick collaboration is needed from Sudan and South Sudan to alleviate the hardship of those stranded in Kosti," Ali Al-Za'tari, the U.N. Resident and Humanitarian Coordinator in Sudan, said.
Between 12,000 and 15,000 South Sudanese are at the Kosti way station, "many of whom have been waiting with their entire house holdings for months for transport to South Sudan," a U.N. statement said.
"All are dependent on collaboration between Sudan and South Sudan and assistance from the international community for proper transportation facilities, food, water, healthcare and other essential services and most do not have the means to transport themselves to South Sudan and are running out of means to support themselves," it said.
South Sudan split from Sudan last year as part of a 2005 peace deal that ended decades of war in Africa's largest nation. The war left 2 million people dead and ended with the peace agreement that included an independence referendum for the south.
Significant issues between the countries remain unresolved, however, including status of their citizens, division of national debt, disputed border areas and sharing of oil wealth.
Tensions peaked this month when South Sudan seized the oil-producing region of Heglig, a resource that fuels the economies of both nations, from its northern neighbor. Heglig oil facilities account for about half of Sudan's production of 115,000 barrels a day.
South Sudanese forces withdrew days later after Sudan lodged protests with the United Nations and African Union, but South Sudan said it continued to come under aerial and ground attack.
South Sudan army spokesman Philip Aguer denied Sudan's claims of aggression, telling CNN Tuesday that their troops have not crossed the border.
"We never crossed the territories of South Sudan," Aguer said. Sudan's charges are "just to cover their own activities."
Aguer accused Sudan of attacking positions in South Sudan's Unity state Tuesday morning. South Sudan forces captured 27 Sudanese solders and three trucks with heavy machine guns, he said.
"This is a serious continuation of acts of aggression from Khartoum," he said.
A spokesman for Sudan's army did not immediately respond to CNN's request for comment.