The United States on Friday warned of a dangerous increase in tensions on the undemarcated border between Sudan and South Sudan after reports of clashes between the two sides.

The United States on Friday warned of a dangerous increase in tensions on the undemarcated border between Sudan and South Sudan after reports of clashes between the two sides.

Washington is "deeply concerned" by the reported incidents in disputed border areas between South Sudan's Upper Nile state and Sudan's Blue Nile, the US embassy in Khartoum said.

These included "reports of Sudanese helicopters crossing into South Sudan and the artillery bombardment by Sudanese armed forces of South Sudanese troops," it claimed.

"Additionally, we are also very concerned about the increase in negative rhetoric, which together with these reports of military clashes is leading to a dangerous increase in tension along the undemarcated border."

Sudan and South Sudan fought along their frontier last March and April.

In September they hailed an end to conflict by signing security and economic agreements but they have not been implemented.