The UN Security Council hopes to vote Wednesday on a resolution that could threaten Sudan and South Sudan with sanctions if they do not stop fighting, diplomats said.
China and Russia, veto-wielding permanent members, are however leading resistance to any warning of international action against the rival countries, which many fear are headed for all-out war.
Sudan on Tuesday warned its southern neighbour, which split away last year, over widening "aggression" as the South alleged fresh clashes despite an African Union peace initiative in the oil-fueled conflict.
Khartoum charged that "South Sudan and its army are working to widen the aggression and occupy some disputed points and areas by force. Sudan cannot allow the occupying troops to impose their power."
More than a week ago South Sudanese soldiers said they had completed a withdrawal from Sudan's main oil region of Heglig, which they occupied for 10 days, while Sudan launched air strikes across the border.
In New York, a resolution drawn up by the United States calls on the two countries to "immediately cease all hostilities" and withdraw troops to their own territory, in line with the call made by the African Union.
The resolution would threaten "additional measures" under Article 41 of the UN Charter, which allows for non-military sanctions.
China, which has strong trade ties with both Sudan and South Sudan, and Russia traditionally oppose warnings of sanctions. And the resolution could change before any vote, diplomats told AFP.
"This time it is less the Russians and more the Chinese," one senior Western diplomat said on condition of anonymity. "If they keep on not liking it, they might abstain. I don't think they will veto."
China is more likely to accept the resolution as the request for possible sanctions has come from the African Union.
"It is much more difficult for the Chinese and Russians to say no to an AU request than a Western plan," the envoy added.
Under the resolution, the two countries would have two weeks to "unconditionally" start talks under AU mediation on borders and sharing oil revenues, and they would have three months to conclude an accord.
UN leader Ban Ki-moon would have to report to the Security Council every two weeks on the crisis.
China and Russia are nervous even though no automatic sanctions are mentioned, diplomats said. "They oppose even the mention of Article 41," one diplomat told AFP.
Speaking after talks with Sudan's Foreign Minister Ali Karti in Moscow on Monday, Russia's Foreign Minister Sergei Lavrov said his country was ready to support the resolution.
"It may include measures of economic pressure. But I would repeat that this is not an automatic decision, but only an intention depending on how the resolution is implemented," he told reporters.
Sudan and South Sudan have kept trading accusations following the Heglig clashes, the most serious in a month of border confrontations.
Sudan's foreign ministry said South Sudanese troops had on Monday occupied a disputed area on the border of Sudan's Darfur and South Sudan's Western Bahr el-Ghazal state.
That came a day after the South took over another disputed point on the Darfur border, the ministry added.
It said the South has "a plan to continue the war and disturb peace and security and stability along the border between Sudan and South Sudan."
Each side denies backing rebels on the other's territory in a proxy war after failing to settle outstanding issues over contested territory, oil revenues and borders.
The South became independent last July after an overwhelming "yes" vote under a 2005 peace deal that ended a 22-year civil war.
On Tuesday, the South's army said Sudanese troops and allied militias clashed with South Sudanese troops along the disputed border.
"Sudanese forces, militias and mercenaries attacked our positions in Hofra in the oil region of Unity state," Southern army spokesman Philip Aguer said, adding that Southern troops had "repulsed the attackers" but were preparing for another expected assault from Sudan.
It was not possible to independently verify the situation.
The British embassy, meanwhile, said Tuesday that it had met with one of its citizens allegedly detained by Sudan's army along the tense southern border and that he was in good condition.
Chris Fielding, the Briton, has been held for at least three days along with Norwegian John Sorbo, South African Thabo Siave and an unidentified South Sudanese.