Sudan has accused Chadian aircraft of bombing its western Darfur region in what it called "repeated aggressions" by its western neighbour, but Chad said its operations against rebels never crossed the border.
Relations between the two African oil producers have been touchy in recent years as both try to quell insurgencies close to their long and porous border. They accuse each other of backing rebels trying to overthrow their respective governments.
"In an unprecedented escalation, Chadian forces have violated the joint border as three Chadian war planes bombed two areas ... in West Darfur ... on December 28th," said a Sudanese foreign ministry statement seen by Reuterstoday.
Chad said its ground and air forces had attacked rebel targets along the border, but had not crossed into Sudan. "The security forces have taken every measure to cleanse the national frontiers of hostile forces," the External Affairs Ministry said in a statement broadcast on state radio in Chad's capital N'Djamena late on Saturday.
"In this context the plane undertook several strictly limited operations along the border against mercenaries who were trying to cross from Sudan. Contrary to the Sudanese allegations, the Chadian army never crossed the border to carry out ground operations on Sudanese territory," it said.
Humanitarian workers are being evacuated from towns along the Sudan-Chad border, two sources in the aid community said, following an escalation of hostilities. Chad accused Sudan on Wednesday of sheltering and re-arming Chadian rebels after fierce battles last month in preparation for a fresh offensive, aiming to further delay deployment of international peacekeeping forces in Darfur and eastern Chad.
It also warned the European Union that further delay in deploying up to 4,000 EU peacekeepers on its border with Sudan, initially due earlier this year and now expected sometime in the coming months, risked "setting the region ablaze".
In Darfur, African Union forces will make what is seen as a largely symbolic handover to a joint UN-AU peacekeeping force tomorrow, although many expect little change on the ground.
Rebels in both Chad and Sudan have profited from their poor relations. Darfur rebels have bases in N'Djamena and a Reuterswitness has seen Chadian rebel camps on the Sudan-Chad border. Chadian army deserters have told Reutersthat they supported Darfur rebels in 2003, when mostly non-Arab insurgents took up arms accusing Khartoum of neglecting the remote region.
Sudan has rejected Chad's accusations that it was helping Chadian rebels, and accused the Chadian government of supporting the Darfur rebel Justice and Equality Movement (JEM) in a recent meeting on Chadian territory in the area of Bahai.
International experts estimate Darfur's war has killed some 200,000 people and forced 2.5 million from their homes.
In eastern Chad militia attacks on villages continue and thousands of desperate Chadians have sought refuge in Darfur and violence has spread to neighbouring Central African Republic.