Scotland's top law officer has met the director of the FBI to discuss progress in the investigation into the Lockerbie bombing.

Lord Advocate Frank Mulholland QC described Friday's talks with James Comey as "very useful" and said they discussed in detail various avenues of inquiry currently being examined.

"Despite the difficulties we remain hopeful that progress will be made," Mr Mulholland said.

The Lord Advocate also met the Libyan ambassador to the UK last week, who, he said, voiced the commitment of the Libyan authorities to help bring others involved in the atrocity to justice.

Libyan Abdelbaset al-Megrahi is the only person to have been convicted of the bombing of Pan Am flight 103 over the south of Scotland on December 21 1988, in which 270 people were killed.

Mr Mulholland will shortly reveal details of his recent high-level meetings during a speech in the United States on the 26th anniversary of the tragedy.

He is to address some of the relatives of those died at a memorial service at the Arlington cemetery in Washington.

Speaking on his third visit to Arlington, Mr Mulholland will say he wants to "demonstrate in action and words our commitment to continuing our work to bring the others involved in the appalling act of terrorism to justice".

He will say: "You know more than anyone else that justice has often been delayed in this case. There have been long periods in the last 26 years when it was not possible to make progress and we have all had to be patient, which is frustrating.

"If I can give you a message today, and it is one I wanted to give you all in person, it is to urge you not to give up hope that others will eventually be brought to justice. We investigators and prosecutors both sides of the Atlantic are as committed to this quest for justice as we have ever been."

Accompanied by Police Scotland's new senior investigating officer in the case, Detective Superintendent Gerry McLean, Mr Mulholland insists his determination to make progress with the ongoing investigation "remains as strong as ever".

He will say: "To reinforce that message, I met last week with the Libyan Ambassador to the UK. He asked me to reassure you that his thoughts are with you at this difficult time and reiterated the commitment of the Libyan authorities to work with us to gather the evidence from Libya to bring the others who worked with Megrahi in the murder of your loved ones to justice.

"I also keep in close contact with the UK ambassador to Libya who is currently based in Tunis and get regular, updates on what is happening. He continues to offer the full support of the UK government to our endeavours, and has been able to get information for us which has been helpful in shaping our approach going forward."

Turning to work with officials in the United States, Mr Mulholland will add: "Our closest ties in this case are of course with our US colleagues with whom we work in partnership. As has been said many, many times, this was, and remains a joint investigation and prosecution effort.

"I met with Director Comey of the FBI on Friday to discuss progress in the inquiry. It was a very useful meeting and the avenues of enquiry currently under investigation were discussed in detail.

"Despite the difficulties we remain hopeful that progress will be made. We reiterated our commitment to work closely together to make progress in Libya and elsewhere; wherever there is an opportunity we will be there. We will follow the evidence relentlessly."

Details of the ongoing investigation have not and will not be made public, the Lord Advocate insists.

"What I can say however is that in addition to the lines of enquiry in Libya there are other lines currently being pursued outwith Libya," he will add.

"We remain cautiously optimistic that these lines of enquiry will bear fruit."

The aircraft was on its way from London to New York when it exploded above Lockerbie.

Megrahi, who was convicted of the bombing and given a life sentence, was controversially released from jail by the Scottish government on compassionate grounds in August 2009 after being diagnosed with prostate cancer and given three months to live.

He died protesting his innocence in Tripoli in May 2012.