THE cold war between Africa's newest neighbours is heating up. South Sudanese troops advanced deep into Sudan on April 10th, capturing its most valuable oilfield, Heglig, in the biggest clash since the south seceded from the north last July. Southern troops claimed to be responding to air and ground attacks from their former master, but the scale of the offensive is unprecedented. A fragile peace process that has survived several bumps in the past few months may now falter. Sudan has suspended its participation in the divorce negotiations in neighbouring Ethiopia. Parliaments in both countries are calling for military mobilisation. The drums of war beat ever louder.

The last strawcould be South Sudan claiming Heglig as its own. A ruling by the Permanent Court of Arbitration in The Hague in 2009 appears to put the field in the Sudanese state of Southern Kordofan. But the south now disputes this. “Heglig is deep inside our borders,” says Colonel Philip Aguer, a spokesman for South Sudan's army, adding that its troops have moved farther north. Sudan will not accept this, and for once it seems to be getting some international support. The African Union is calling on the south to withdraw its soldiers immediately and unconditionally. Sudan has complained to the UN Security Council.

The crisis is a direct result of both sides' failure to make progress in negotiations over post-secession security arrangements, citizenship rules and oil revenues, among other issues that should have been resolved long ago. Both countries have accused each other of supporting rebels on their territory since before separation. Of the two, the southern rebels in Sudan are by far the stronger. Known as SPLM-North, they supported the decades-long southern fight for independence but found themselves on the wrong side of the border at separation. The group controls much of the Nuba mountains in Southern Kordofan and launches guerrilla raids in Blue Nile state. Sudan says SPLM-North is getting weapons and supplies from South Sudan, and that its fighters go there to rest after battles. The northern rebels in the south are smaller but have sometimes caused havoc in Unity and Upper Nile states. A local oil worker says they previously helped to defend Heglig.

Just as Sudan faces a renewed threat from the south, the long-running civil conflict in its western Darfur region is escalating again. Three years ago, General Martin Agwai, then commander of African Union peacekeeping troops in Darfur, said the conflict was “over” and that banditry was now the biggest problem. But on April 3rd areas around Sortony in North Darfur were hit by aerial bombardments and attacked by pro-government militias on the ground, forcing thousands of civilians to flee and sparking fears that the bad old times are back.

They may be. A dissident report by former UN investigators that has been submitted to the Security Council—but not yet published—documents the recent recruitment of non-Arab militias by the Sudanese Armed Forces. They are accused of ethnic cleansing of the Zaghawa tribe,which is led by Minni Minnawi, a Darfuri rebel who last year withdrew from a peace agreement that had made him a presidential adviser. The report says the use of non-Arab militias marks a “significant evolution”. At least 70,000 civilians appear to have fled new attacks in 2011.