The much-heralded agreement signed last month by South Sudan's government and rebels was meant to do what at least seven previous accords had not: end the country's brutal 20-month civil war, which has claimed more than 50,000 lives.

But less than a week after President Salva Kiir signed the peace deal, UN officials have confirmed significant military action is being carried out by the government, including the deployment of helicopter gunships, to attack rebel positions.

The UN's mission in South Sudan, UNMISS, reported on Wednesday that two attack helicopters fired rockets at rebel positions on the west bank of the Nile in Malakal, an oil hub in Upper Nile State that has changed hands countless times during the war.

"Heavy explosions, including mortar shells, as well as heavy artillery and small arms fire were also heard coming from Malakal towards areas located on the west bank," said UN spokesperson Stephane Dujarric the following day.

On Friday, a spokesperson for the rebel South Sudan People's Liberation Army In Opposition (SPLA/IO) said in a statement sent to VICE News, "Four government gunships carried out aerial bombardment for the second day on our bases in Warjok and Detang" on the west bank of Malakal.

"Now, they are extending their military offensive on our positions southward, 30 minutes from Lelo, where they pushed back our forces two days ago when they launched four Helicopter gunship airstrikes," added the spokesman, Colonel William Gatjiath Deng.

Rebel leader Riek Machar agreed to the terms of the deal on August 17 in the Ethiopian capital, Addis Ababa. Kiir, who once called Machar his vice president, initially refused and returned to South Sudan.

Under intense international pressure, including the threat of a UN Security Council resolution authorizing an arms embargo and new sanctions, Kiir signed the deal on August 26. But Kiir also made clear that he had serious qualms about the agreement, and many observers questioned whether peace would take hold. Despite concerns, the US, which had authored the draft resolution meant to push Kiir to sign, stopped pushing for the text's passage upon his signature.

On Friday, following the latest reports of fighting, the Security Council met again on South Sudan. In a closed session, UNMISS chief Ellen Margrethe Loj briefed the council by video conference, confirming that attacks had taken place in the vicinity of Malakal, according to diplomats.

Reached by phone on Friday afternoon, a staff member at South Sudan's UN mission said they had not received information about the violence, including whether any government attacks were retaliatory in nature. As confirmed, the fighting is a clear violation of the ceasefire, which took effect on August 29.

Deng, the SPLA-IO spokesperson, said that the "continual attacks may derail the peace process signed on 17 August."

Civil war broke out in South Sudan in December 2013, less than three years after the country achieved independence from Sudan. Details of the power struggle in the war's early days remain murky. Kiir accused Machar of attempting to mount a coup; Machar, who was booted from Kiir's cabinet earlier in the year, denied this, but was soon the nominal leader of a large rebel force, much of which had broken off from the national army. Kiir is a member of the country's Dinka community, Machar is a Nuer, and fighting has often, but not always, broken along ethnic lines.

After nearly two years of war, concerns mounted that even when a peace deal was signed, fragmentation on both sides would leave each man unable to control hardline elements in their camps. Kiir in particular was reportedly urged by several generals to not sign the deal, but he relented.

After the council session Friday, Russian Ambassador Vitaly Churkin, the Security Council's current president, told reporters that the council was ready to "consider appropriate measures to ensure the full implementation of the agreement, without exception."