U.S. diplomats are expressing “deep concern” over violence on the border of Sudan and South Sudan, at the end of a visit to the region.

The U.S. special envoy to Sudan, Princeton Lyman, and U.S. Deputy National Security Advisor Denis McDonough visited Khartoum and Juba for talks on Sunday and Monday.

In a statement Monday, the White House said the two discussed recent cross border attacks in the states of Southern Kordofan and Blue Nile, and the resulting humanitarian crisis in the border region.

The White House said the diplomats told Sudanese officials in Khartoum that the humanitarian crisis is exacerbated by Sudan's blocking of international aid to civilians in the region.

While in Juba, the diplomats told South Sudanese officials that the south must respect the sovereignty of Sudan, including ending its support of armed groups in Southern Kordofan and Blue Nile states.

Earlier this month, armed groups in Sudan announced a military alliance that has the stated goal of overthrowing the government of President Omar al-Bashir.

Sudan has accused South Sudan, which became independent in July, of helping set up the alliance. South Sudan denies the charges.

Sudan and South Sudan have been almost constantly at odds since the south became independent in July.