UN human rights chief Navi Pillay has called for an investigation into the bombing of a refugee camp in South Sudan, which became independent in July. The UN has accused Sudan of carrying out the attack in oil-rich Unity state, just south of the border. Khartoum, which is fighting pro-southern rebels on its side of the frontier, denies the allegation. South Sudanese officials say at least 12 people were killed in the air raid. In other developments: * UN peacekeeping chief Herve Ladsous briefed the UN Security Council about the bombing, which he blamed on Sudan, the Reuters news agency reports; * South Sudan's President Salva Kiir accused Sudan of planning an invasion; * The US called on Mr Kiir's government to show restraint * US campaign group Enough Project says satellite images show that Sudan is strengthening its bombing capabilities along the border, raising the prospect of more attacks. Ms Pillay said there needed to be an independent and credible investigation into Thursday's bombing of the Yida camp, which was witnessed by the BBC's James Copnall. "If indeed it is established that an international crime or serious human rights violation has been committed, then those responsible should be brought to justice," she is quoted by Reuters as saying. However, Sudan Armed Forces spokesman Sawarmi Khaled Saad vehemently denied any links to the raid. "This information is completely false. We didn't bomb any camps or any areas inside the borders of South Sudan," he told the AFP news agency. Thousands of people have fled fighting in Sudan's South Kordofan region and crossed the border to take refuge at the Yida camp. Sudan's army has often been accused of bombing South Kordofan, where it is facing a rebellion from pro-southern groups. The South Sudan Liberation Army rebel group is also active in Unity State - it denies claims that it is working for Khartoum in order to destabilise its neighbour. Miabek Lang, the commissioner of Pariang county in Unity state, said at least 12 people had been killed and 20 wounded in the bombing. The BBC's James Copnall in Unity state says he arrived at the camp at about 12:00 GMT on Thursday as a second round of bombs hit. He says that just as a UN helicopter, carrying food aid, settled on to a makeshift landing zone at the camp, there was a deep and terrifying thud of a nearby explosion. A large plane was spotted heading to the north, our reporter says. Several residents of the refugee camp told him it was an Antonov plane, often used by the Sudanese government as a makeshift bomber. The refugees said the plane had circled before launching two bombing raids. Five bombs were dropped, of which four exploded, they told our correspondent. Incidents like this, and both sides' belief that the other is sponsoring rebels on their territory, are contributing to a rapidly deteriorating relationship between Sudan and South Sudan, he says. South Sudan's independence in July was the culmination of a peace deal that ended a decades-long north-south civil war.