South Sudan's foreign minister has warned his country is on the brink of war with Sudan following days of fierce fighting along the border.
Nhial Deng Nhial told the BBC Sudanese forces had invaded the town of Jau, which was in the south.
He urged the international community to intervene and said he hoped full-scale hostilities could still be avoided.
South Sudan seceded from the north in July following years of civil war in which some 1.5m people died.
The border between the north and south has not yet been officially designated.
Since July Khartoum and Juba have accused each other of supporting rebels in the border areas.
'Tanks and aircraft'
Mr Deng Nhial said the clashes in Jau, which he said was a town in Unity state, were the biggest threat to peace since South Sudan's independence.
"Although there have been frequent aerial bombardments of different places in the Republic of South Sudan, we think that Khartoum has raised this offensive to an entirely new level by committing ground forces to cross into the Republic of South Sudan," he told the BBC's Focus on Africa programme.
"We are still very much committed to the principle of dialogue with Khartoum - we are still hopeful that we can pull back from the brink of outright war."
Earlier, Col Philip Aguer, spokesman for South Sudan's army - the South People's Liberation Army (SPLA) - told the BBC that Khartoum had used tanks and long-range artillery in the offensive on Jau, which started on Saturday.
Antonov aircraft had also bombed the area, he said.
Southern troops had now recaptured the town, but Sudanese soldiers were still in South Sudan, he said.
"This is a war situation and if they don't withdraw, the SPLA will force them out," Col Aguer told the AFP news agency.
Across the border in the state of South Kordofan, Sudan's army has for several months been battling rebels, who once fought against Khartoum during the civil war.
South Kordofan is one of several border areas which failed to hold popular consultations about their future ahead of South Sudan's independence.
Mr Deng Nhial denied accusations that his government was supporting the rebels in the northern border areas, known as the SPLM-North.
"We had been associated with the SPLM-North during the years of our struggle. After independence we severed all military ties with our units in the north and we didn't provide any additional equipment," he said.
The foreign minister said it was important that the border be properly demarcated.
"Khartoum continues to drag its feet although we are agreed that close to 80% of the border is no longer in dispute," he said.
"This in our mind raises serious suspicions."