Liberian rebels recently held talks with Northbridge Services Group, an Anglo-American private military company, about a plan to arrest President Charles Taylor (pictured) and take him to a United Nations-backed court in Sierra Leone, according to parties to the discussions.

As international peacekeeping forces begin to deploy in the war-torn west African nation, the FBI is investigating attempts by Northbridge to work for the rebel group, Liberians United for Reconciliation and Democracy (Lurd), despite the existence of a United Nations arms embargo, according to a company director and US official.

Company directors also confirmed on Wednesday that UK Customs had investigated whether its UK arm had violated UN arms embargoes. UK Customs declined to comment about its role.

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The overlapping investigations suggest the US and UK authorities are acting to stop western mercenaries getting involved in Africa's interlocking conflicts. In the UK there is deep sensitivity surrounding the issue of mercenaries following a 1997 scandal in which the government was accused of using Sandline International, a British company, to circumvent UN arms embargoes. That operation was uncovered by UK Customs.

This year the British government spoke out publicly against Northbridge after reports that it was hiring several hundred fighters for the government in Ivory Coast. Jack Straw, the foreign secretary, said at the time that the deployment by Northbridge would "seriously undermine the peace process".

Now it has emerged that Northbridge has been holding talks with Liberian rebels over a possible role in their own conflict.

For several months, Northbridge has been trying to get the UN-backed special court in Sierra Leone to fund the planned operation to arrest Mr Taylor, who has been charged with war crimes. The court said it was open to the Northbridge operation, but that it had no funds and privately suggested the US might be willing to pay for it.

As fighting intensified, Northbridge made a further offer to deploy up to 2,000 men to "enforce" peace in Liberia ahead of the arrival of a UN peacekeeping force. That is an offer that the Liberian rebels have been keen to take up, envisaging an operation that would involve the arrest of Mr Taylor.

"Lurd would wholeheartedly welcome Northbridge Services to come and help stabilise the situation, not only in Liberia but also the entire region," said Bodioh Siapoe, a Lurd spokesman in the US. Northbridge has previously admitted to links with Lurd as well as T.Q. Harris, the rebel group's California-based negotiator. Mr Harris, a former Liberian presidential candidate, pointed out that until this week Northbridge had been the only group offering to halt the bloodshed.

"The basis for this [negotiation] was desperation," he said, noting that at the time the plan was being considered neither Liberia's neighbours nor the US appeared willing to deploy peacekeepers. "There was a need for an alternative. . . We started looking at options and thought about the need for a private army."

Mr Harris said his opposition group had started raising funds to hire Northbridge, but had stopped because of the deployment of Nigerian peacekeeping troops, who began arriving in Liberia earlier this week. But he said the plan had not been shelved.

The company's UK principal, Andrew Williams, said he had been questioned by the US embassy in London as part of an FBI investigation. The embassy declined to comment, although one US government official confirmed the probe and said the FBI was concerned about whether the involvement of Northbridge in Liberia would breach the UN arms embargo.

"It came up. . . that Lurd was trying to hire these guys," the official said. "Some people notified the FBI. Now they [Northbridge] are being investigated."

The US probe appears to have split Northbridge, a company set up by former US and UK soldiers, which has seen bitter infighting between its two principals.

Northbridge claims to be "probably the largest [company] of its type in the world" with "staffing from organisations such as the US Central Intelligence Agency and US and British special forces". It recently acquired a Ukranian company that provides Ukranian and Russian personnel, according to Pasquale DiPofi, the company's US principal.

According to Mr Siapoe, Northbridge was originally going to offer services to the government of Mr Taylor but was later persuaded to change sides.