The American ambassador to Libya sent a diplomatic cable spelling out his worries about the security situation in Benghazi on the day he was killed in a raid on the US consulate, it has been revealed.

Chris Stevens told State Department officials that two militias that had been providing security in the city were threatening to withdraw over a dispute about US policy.

The news comes at the same time as it has emerged that the head of a special-operations team helping the embassy in Libya with security asked for 'more, not less' reinforcements before the government pulled dozens of people out of the country earlier this year.

On September 11th, the day he died, Mr Stevens wrote to Washington officials detailing a dispute involving the leaders of two prominent Benghazi militias who were responsible for security in the city.

The two men, Wissam bin Ahmed and Muhammad al-Gharabi, claimed that the US was lobbying for centrist politician Mahmoud Jibril to become Libya's Prime Minister.

They said that if he won the vote, they 'would not continue to guarantee security in Benghazi, a critical function they asserted they were currently providing,' according to Mr Stevens.

Despite that warning, Mr Stevens did not ask for more US troops, and commented that Benghazi officials believed the city was becoming safer.

The cable made no mention of a US-made YouTube video mocking the Prophet Muhammad (PBUH) that was originally thought to have been the motivation for the deadly assault on the consulate later that night.

The American compound was being guarded by members of the 'February 17 Martyrs Brigade', a militia that shared members with the groups run by Mr bin Ahmed and Mr al-Gharabi.

It was not only Mr Stevens who saw potential security issues cropping up in Libya before the September 11 raid.

Lt. Col. Andrew Wood, who was head of a Special Forces 'Site Security Team' in the country, said officials felt “like we were being asked to play the piano with two fingers” after a number of troops were withdrawn from Libya in August.

He told CBS This Morning that worried embassy staff had approached him to ask if they would still be safe when his team had left.

“I could only answer that what we were being told is that they're working on it,” he said.

He added: “Shooting instances occurred, many instances involved the local security guard force that we were training.

“Constantly, there were battles going on between militias, criminal activity and that became increasing danger as time went on as well.”

Mr Wood claimed that other senior officials, including Mr Stevens, had requested a boost in the U.S. security presence, saying: “We felt we needed more, not less.”

Although his team was based in the city's capital Tripoli, Mr Wood said he would have accompanied the ambassador to Benghazi had he still been in the country.

State Department officials said that as the Site Security Team was intended to help re-open the embassy in Tripoli, their departure from Libya was irrelevant to the subsequent security situation.

They also claimed that Mr Wood did not know the details of the situation in Benghazi, which is 640 kilometres from the capital.

Mr Wood is one of many witnesses who will this week appear at a hearing of the House Oversight Committee investigating the assault.