Updated 1.34pm: The Islamic State group has released a video purportedly showing the execution of some 30 Ethiopian Christians captured in Libya.

Addis Ababa condemned the reported killings on Sunday and said its embassy in Egypt was trying to verify the video to ascertain if those murdered were indeed Ethiopians.

"We strongly condemn such atrocities, whether they are Ethiopians are not," Ethiopian Communications Minister Redwan Hussein told AFP.

The White House has condemned the executions as "brutal mass murder".

The US Government has called for stability in Libya, which has been mired in political chaos and unrest since the US backed 2011 uprising that toppled former strongman Muammar Gaddafi.

The 29-minute IS video purports to show militants holding two groups of captives, described in text captions as "followers of the cross from the enemy Ethiopian Church".

A masked fighter in black brandishing a pistol makes a statement threatening Christians if they do not convert to Islam.

The video then switches between footage of one group of about 12 men being beheaded by masked militants on a beach and another group of at least 16 being shot in the head in a desert area.

It was not immediately clear who the captives were or how many were killed.

Before the killings, the video shows purported footage of Christians in Syria, saying they had been given the choice of converting to Islam or paying a special tax, and had decided to pay.

The video bore the logo of the IS media arm and was similar to past footage released by the jihadists, including of 21 Coptic Christians beheaded on a Libyan beach in February.

Feeding on the political chaos and unrest that has wracked Libya since the 2011 uprising that toppled dictator Muammar Gaddafi, several Libyan jihadist groups have pledged allegiance to IS.

Addis Ababa says IS, which seized chunks of Syria and Iraq and won support of jihadist groups across the region despite a reputation for brutality, has also gained a foothold in Ethiopia.

"There are elements of IS around Ethiopia who are already carrying out operations, even though under a different name," said Redwan, referring to the al-Shabab group.

"We will keep on fighting them."