The United States on Thursday denounced as "unacceptable" reported Sudanese government air strikes inside the territory of the new state of South Sudan.

South Sudan officials said Sudanese fighter jets have bombed oil and water wells deep inside South Sudan and its ground troops have crossed into contested oil-rich border regions, but Sudan denied the claims.

"The United States is alarmed over reports of aerial bombardments by the Sudan Armed Forces inside South Sudan," State Department spokeswoman Victoria Nuland said in a statement.

"Such incidents are unacceptable and threaten to escalate tensions between the two states," she added.

Nuland also said "continued aerial attacks on civilian targets by the Sudan Armed Forces are deplorable and constitute violations of international law for which there must be accountability.

The United States, she said, "demand that the government of Sudan end these aerial bombardments and immediately allow humanitarian access to civilians in Southern Kordofan and Blue Nile," which are inside Sudan.

Oil-producing South Kordofan remained under Khartoum's administration when South Sudan became independent in July, but fighting since June has pitted Nuba rebels, once allied to rebels in the South, against the Sudanese army.

A similar conflict is taking place in Blue Nile state.

US Secretary of State Hillary Clinton on Wednesday accused Sudanese President Omar al-Bashir of trying to undermine the new South Sudan, adding Washington would consider increasing pressure on Bashir to reverse course.

Fearing the potential of a new war, the United States is calling for full implementation of the 2005 Comprehensive Peace Agreement, which ended two decades of war between northern and southern Sudan.

The agreement led to a referendum which resulted in the creation of a separate state in South Sudan last July.

Tensions have been raised by the still undemarcated border, parts of which cut through oilfields, as well as mutual allegations that each side backs rebel forces against the other.