KHARTOUM  - Sudanese President Omar al-Bashir on Thursday vowed to teach “a lesson by force” to the South Sudanese government over its seizure of the north’s main Heglig oil field.

His latest threat came as diplomatic efforts to end the deadly standoff intensified. Keeping up war rhetoric that has sparked concern from the United States, Bashir dismissed talk by the United Nations that sanctions might be imposed on South Sudan.

“United States will not invoke sanctions on them, and the (UN) Security Council will not, but the Sudanese people are going to punish them,” declared Bashir, who holds the rank of field marshal and wore desert camouflage and a beret to address a rally of freshly-trained paramilitary troops.

On the battlefield, the Southern army repulsed ground attacks in a widening conflict zone along the border, said the South’s military spokesman Philip Aguer.

Ground assaults and air strikes were reported in the Heglig region, as well as three Southern border states, he said, adding the Southern army was “still in its positions”.

Sudan has vowed to reclaim Heglig “by hook or crook”, but its military has released virtually no information about the situation on the ground.

Rebels from Sudan’s Darfur region said they seized two Sudanese military positions 40 kilometres (25 miles) north of Heglig. The insurgents, from the Justice and Equality Movement, deny fighting alongside the South Sudanese.

AFP reporters who have visited the Heglig battle zone along the disputed border say dead bodies and destroyed tanks were strewn about.

East Africa’s main diplomatic body, the Inter-Governmental Authority on Development (IGAD), expressed “grave concern about the escalating conflict”.

The IGAD, which mediated peace talks that ended Sudan’s 1983-2005 civil war in which some two million people died, said it was ready to provide “all possible assistance to maintain peace and stability.”

On Tuesday, the Security Council discussed possible sanctions against both Sudan and South Sudan in a bid to halt a wider war.

Clashes escalated last week with waves of air strikes hitting the South, and Juba’s seizure of the Heglig oil hub on April 10.

“We will teach them a lesson by force,” Bashir said of the South Sudanese government. “Heglig is not the end. It is the beginning.”

South Sudan’s Information Minister Barnaba Marial Benjamin responded in Juba that his country was not at war with Khartoum and was interested in peaceful relations.

“The Republic of South Sudan considers Sudan as a neighbour and friendly nation, not an enemy,” Benjamin told reporters.

The US on Wednesday voiced concern and called for an immediate halt to the fighting after Bashir threatened to topple South Sudan’s “insect” government.

Bashir’s latest remarks came as Princeton Lyman, the US special envoy on Sudan and South Sudan, visited Khartoum in an effort to ease tensions.

China, which has a vested interest in the oil industries of both nations, also reiterated its concern over the escalating conflict, calling for a resumption of dialogue.

Sudan held talks on Wednesday with African Union chief Jean Ping and Ethiopian Prime Minister Meles Zenawi as part of its diplomatic efforts to pressure South Sudan into a pullout from Heglig.

Khartoum’s parliament has declared a “mobilisation and alert” of the population and this week voted unanimously to brand the South Sudanese government an enemy.

Benjamin, the Southern minister, said the move amounted to a “declaration of war”. Sudan was using Heglig as a base to launch attacks on his country, he alleged.

The UN, the US and the EU have criticised the South’s occupation of Heglig but have equally denounced Sudan’s air strikes against the South.

There are widespread fears that the fighting will spread. It is already the worst since South Sudan’s independence last July.

Although South Sudan disputes it, Heglig has been internationally regarded as part of Sudan.

Since the invasion, production at Heglig has been shut and facilities there are leaking. The field accounted for about half of Sudan’s oil, and its loss has worsened an economic crisis.

The International Monetary Fund on Wednesday forecast that Sudan’s economy will shrink 7.3 percent this year, while consumer prices are expected to rise by an average of 23.2 percent.