Millions of dollars of arms shipments have flooded South Sudan since civil war broke out eight months ago, weapons monitors said Tuesday, with countries key to peace also involved in the supply.

The shipments are prolonging a conflict in which thousands of people have been killed and more than 1.5 million have been forced from their homes, monitors said.

Rights groups including Amnesty International and Human Rights Watch have called for an arms embargo, while the UN Security Council has repeatedly warned of possible sanctions.

"Since the start of the conflict there has been an influx of weapons into the country compared to the post-independence period" of the last three years, said Jonah Leff, who heads Conflict Armament Research, a group that tracks weapon flows.

"In particular these flows have been more expensive, and more sophisticated," with shipments including anti-tank weapons, Leff told AFP.

Weapons experts and Amnesty said they have confirmed shipments totalling $38 million (28 million euros) worth of weapons including anti-tank missiles, grenade launchers and assault rifles.

The weapons were bought from China before fighting began, and delivered to land-locked and oil rich South Sudan via Kenya in June.

However, more arms have been supplied.

"It is not the tip of the iceberg, but there is a sizeable chunk more," Leff added.

At the same time China, a member of the Security Council and a major player in South Sudan's oil fields, is supporting peace talks in Ethiopia mediated by east Africa's IGAD bloc.

And IGAD nations bordering South Sudan are also involved, with Uganda sending in troops to support Juba's government.

Guns and ammunition from Sudan, one of Africa's top arms manufacturers, have been used by all sides in the conflict. Shipments of arms have also transited through Kenya.

While rebels are not believed to have received large scale shipments of guns -- many are mutinous soldiers who looted army supplies -- there is considerable cross-border trade in bullets.

"Ammunition needs constant replenishment," Leff said.

Both the government and rebels agreed not to rearm as part of three failed ceasefire deals, pacts broken each time within hours of signing.

Unhindered weapons flows will "fuel further atrocities, and be used in violation of international human rights and humanitarian law," said Amnesty's Elizabeth Deng.