Major General Mac Paul, the deputy director of military intelligence for South Sudan, said on Monday that two MiG 29 fighter planes dropped three bombs, two of which landed near a bridge that connected Bentiu, the capital of Unity State and Rubkona, leaving one person dead and three others wounded, Al Jazeera reported, citing a military official.

“This is a serious escalation and violation of the territory of South Sudan. It’s a clear provocation,” Paul said. “We are building up troops because we think that the Sudanese army is also building up.”

International journalists saw at least two planes drop bombs and a huge plume of smoke rising from a market, where they saw the body of a young boy, reports BBC.

Two fighter jets fired four missiles at the neighboring towns, divided by a river, hitting an open air market and killing at least one person, said journalist Alan Boswell, who is in Bentiu. Boswell was in his car crossing the bridge between the two towns when anti-aircraft fire erupted. That was followed, he said, by missile strikes.

“I saw one boy who about 10-years-old, who was completely burned,” he said by telephone. “There are other casualties.”

The remains of thatch-covered stalls in Rubkona smoldered for hours after the bombing, which occurred in the morning, Boswell said.

In the hours following the airstrikes, Boswell said the military presence in the area increased. Two South Sudanese generals who were arriving near the towns said the airstrikes were “a clear escalation” by Sudan, he said, reports CNN.

Since June 2011, heavy fighting between the Sudanese armed forces and the Sudan People’s Liberation Movement-North in South Kordofan and Blue Nile states has driven tens of thousands of Sudanese refugees into Ethiopia and the newly independent South Sudan, states the UNHCR.

Sudan and South Sudan have been drawing closer to a full-scale war because of the unresolved issues of sharing oil revenues and the disputed border.

The United Nations Mission in South Sudan (UNMISS) today deplored the continued aerial bombardments in Bentiu town in the country’s Unity state, and called on the country and its neighbour, Sudan, to take all measures to ensure the safety of civilians. “These indiscriminate bombings resulting in the loss of civilian lives must stop,” said the Special Representative of the Secretary-General for South Sudan, Hilde F. Johnson. “I remind the parties to the conflict of their obligation to abide by international human rights and humanitarian law, take all measures not to harm civilians, and guarantee the safety of international aid organizations and United Nations personnel and assets,” Ms. Johnson said.

Last week, top UN officials spoke out against bombings by Sudanese forces in the cities of Mayom and Bentiu in South Sudan which killed at least eight civilians and wounded 22 others.

Secretary-General Ban Ki-moon also reiterated his deep concern over continuing hostilities between the two countries, and urged their governments to halt the escalating violence along their border areas.