UN Security Council diplomats went behind closed doors Friday to discuss continued fighting in South Sudan and weigh whether to impose an arms embargo and sanctions.

The 15-member council was meeting at the request of the US after Secretary of State John Kerry warned President Salva Kiir to respect an oft-broken ceasefire.

Under a peace agreement signed by President Kiir and rebel leader Riek Machar, a ceasefire was due to enter into force last weekend but fighting has continued in Upper Nile State.

British ambassador Matthew Rycroft told reporters that "the council will need to respond" to the violations of the peace accord, the latest in a string of agreements aimed at ending the 20-month war.

READ: South Sudan truce threatened as govt, rebels trade accusations

The world's youngest nation, South Sudan descended into bloodshed in December 2013 when President Kiir accused Dr Machar, then his deputy, of planning a coup.

The violence has left tens of thousands of people dead and the impoverished country split along ethnic lines.

Panel of experts

The US has presented a draft UN resolution imposing an arms embargo and targeted sanctions on individuals who block provisions of the peace accord, but the measure has yet to come up for a vote.

The text calls for the arms embargo to take effect on Sunday along with a travel ban and an assets freeze on "individuals, including the senior political leaders of the government of South Sudan, as well as individuals or entities that violate the terms of the ceasefire."

Angolan ambassador Ismael Gaspar Martins said he supported an arms embargo, a measure he said was overdue.

"The council should exert pressure for the agreement to be fully implemented by all parties," he told reporters.

A recent report by a UN panel of experts detailed arms supplies from China to government forces along with shipments of Russian-made and Israeli-made weapons, possibly supplied through regional countries.