By Simon Cameron-Moore

ISTANBUL (Reuters) - Afghanistan and Pakistan have a chance at talks in Istanbul to end a blame game over a series of militant attacks that have deepened their mutual mistrust, officials from the Turkish host government said on Monday.

Presidents of the three countries meet on Tuesday as Afghanistan enters a critical phase in its transition, with the United States planning to pull its combat troops out by the end of 2014, and some Western countries already withdrawing theirs.

Relations between Islamabad and Kabul have been plagued by regular bouts of recriminations during the decade-old Taliban insurgency, with Afghan officials publicly airing suspicions that Pakistani intelligence is supporting the Taliban and the Haqqani network, an insurgent group allied to the Taliban.

Noting a deteriorating regional environment, a Turkish official said: "Now is perhaps the time to try to reverse the course."

"We sense that they have a genuine wish to talk to each other because they realize this trend is not helping either of them," the official said before the summit of the three presidents, Afghanistan's Hamid Karzai, Pakistan's Asif Zardari and their Turkish host Abdullah Gul.

There has been a flurry of speculation about drawing the Taliban and Haqqani group into negotiations to end the fighting in Afghanistan.

Underlining suspicions that Islamabad is backing the Taliban, Karzai has said he should be talking to Pakistan in any negotiations.

Significantly, Pakistan's military chief General Ashfaq Kayani will meet his Afghan counterpart on the sidelines of the what will be the sixth summit between the three leaders.

The tri-lateral meeting will be followed by a regional conference on Afghanistan to be attended by U.S. Secretary of State Hillary Clinton, Indian Foreign Minister S.M. Krishna and their counterparts from France and Germany, among others.

Many Afghans believe Pakistan is supporting the Taliban in order to regain influence in Kabul once Western forces leave.

A war of words escalated after the assassination in Kabul on September 20 of former Afghan president Burhanuddin Rabbani, who was heading a peace commission.

Afghan officials believe the suicide attack was ordered by Taliban leaders who, they say, are based in the southwestern Pakistani city of Quetta and that the bomber was Pakistani.

Pakistan has also come under intense pressure from Afghanistan and the United States over allegations that its military's Inter-Services Intelligence Directorate (ISI) has close ties with the Haqqani group.

The Haqqani group, which operates within Afghanistan, with a rear base in the Pakistani tribal area of North Waziristan, has been blamed for a string of high profile attacks including one on the U.S. embassy in Kabul in September.

Pakistan has vehemently denied the allegations, and officials accuse Afghanistan of deflecting attention away from its own failures. They have also called on Afghanistan and U.S. forces there to act decisively against anti-Pakistan militants operating from Afghan territory.

Pakistan's discomfort became more acute when Karzai signed a strategic partnership with its arch-rival India earlier this month, stoking old Pakistani fears of encirclement by unfriendly neighbors on its western and eastern borders.

Turkey, a Muslim member of NATO, hopes both sides will speak frankly at the Istanbul meeting to overcome misunderstandings. The hosts want to revive some spirit of cooperation with an agreement they hope will be signed in Istanbul.

"We are also hoping to have those two countries sign modest cooperation protocols," the official said. Details had to be sorted out. "But we hope to be able to get them into that state of mind," he said.

The later conference, which will be attended by 14 countries from the region and 13 involved in helping Afghanistan, is also expected to agree on a document that will contain confidence building elements, he said.

(editing by David Stamp)