A senior UN official has started his investigation into the killing of opposition demonstrators in Guinea.

UN Assistant Secretary General Haile Menkerios is to investigate the events of 28 September, when Guinean soldiers opened fire on the protesters.

They were calling on Guinea's military ruler not to stand for election.

A BBC correspondent says Mr Menkerios' arrival is the most powerful sign yet that the world beyond Africa intends to pursue the case.

Human rights groups say some 157 people died in last month's clashes, but the junta puts the toll at 57.

On Sunday, Mr Menkerios met the military ruler, Capt Moussa Dadis Camara, Guinean Prime Minister Kabine Komara, and members of the opposition.

The UN investigator said the prime minister assured him that Guinea's military government welcomed the probe and would co-operate fully.

The Guinean authorities have blamed the September killings on out-of-control soldiers and opposition provocation.

Capt Camara has himself called for an investigation.

BBC West Africa correspondent Caspar Leighton says the arrival of the UN team in Conakry adds to the broad international pressure being put on the military government to write itself out of Guinea's political future.

So far Capt Camara has yet to yield, despite an African Union deadline for him to do so.