The presidents of Pakistan and Afghanistan have been holding their sixth trilateral summit with their Turkish counterpart in Istanbul. The meeting between the Pakistani and Afghan leaders is the first since the assassination of Afghanistan's former president and chief peace negotiator, Burhanuddin Rabbani, for which Kabul holds Islamabad responsible.

Resolving the ongoing Afghan-Pakistani tensions over the assassination of former president Rabbani was a key issue during the day of talks hosted by Turkish President Abdullah Gul. During a joint press conference with Presidents Asif Ali Zardari of Pakistan and Hamid Karzai of Afghanistan, Gul said a breakthrough had been achieved.

He said Pakistan and Afghanistan have reached an important agreement. He said they have agreed to cooperate on contributing to the national investigations regarding the assassination of Rabbani. President Gul said the investigation will be coordinated by the two countries' foreign ministries and will involve their intelligence services.

Rabbani, who was seeking to negotiate with the Taliban to end their insurgency, was assassinated by a suicide bomber. His killing has been the latest source of deep distrust in the already difficult relations between Pakistan and Afghanistan. President Karzai has accused the Pakistani intelligence service (ISI) of being involved in the assassination, a charge that Islamabad has denied.

Karzai welcomed the agreement.

"I hope that the mechanism that we have established for pursuing the matter of the assassination of president Rabbani [meets] with success, and will lead us to a more fruitful and in-depth continuation between Afghanistan and Pakistan, inshallah (God willing), into the future," said Karzai.

The Afghan president has recently been increasingly vocal in charging that the ISI is giving support to the Taliban insurgency, a charge Islamabad also strongly denies. But in another step to defuse tensions, the heads of the two countries' armed forces and intelligence services also held talks in Istanbul, along with their Turkish counterparts.

Reducing bilateral tensions is seen as key to improving regional stability and ensuring Afghanistan's future. On Wednesday, Afghanistan and Pakistan will participate in a regional foreign ministers' summit on development and aid, again hosted by Turkey.

Observers say creating a positive political momentum in relations between the two countries is important ahead of international meetings on Afghanistan set to take place in Bonn, Germany, this December, and in the U.S. city of Chicago next year.