The Sudanese army said it recaptured several areas bordering South Sudan on Tuesday as fighting raged across the frontier between its ex-rebel rulers and a one-time civil war ally.
The rebel Sudan People's Liberation Army-North (SPLA-N), which has been fighting government troops in border states since 2011, denied the army's claim of gains in South Kordofan.
An armed forces spokesman quoted by state media said that troops had penetrated the Al-Ardiba area in the eastern part of the Nuba mountains, a region whose non-Arab population's civil war sympathies largely lay with the now independent south.
Troops suffered several dead and wounded in the operation but seized "more than 30" military vehicles, the statement said.
The rebels denied losing any positions in the state, where they took up arms soon after South Sudan voted nearly three years ago to break away.
"This... is completely false. We are holding our positions and we have not evacuated any of them," SPLM-N spokesman Arnu Lodi told AFP.
"It is true that there are ongoing clashes in the area, but no side controls the region," he added.
Sudanese authorities have severely restricted access to the war zone for aid workers, journalists and foreign diplomats, making it difficult to verify claims.
The Sudanese government said more refugees from the fighting in neighbouring South Sudan had fled into South Kordofan on Tuesday.
Fighting erupted in South Sudan more than two weeks ago after Riek Machar, who was sacked as vice president in July, was accused of attempting a coup against President Salva Kiir.
Machar was for a time an ally of Khartoum in the 1983-2005 civil war that resulted in the south's secession.