* Taliban claim attack on soldiers in eastern Kunar province
* Six soldiers missing after pre-dawn assault
* Taliban suspends efforts to arrange exchange of prisoners (Adds additional comment from Karzai)
By Mohammed Anwar
ASADABAD, Afghanistan, Feb 23 (Reuters) - The Afghan Taliban killed 21 soldiers in an assault on Sunday in a remote mountainous region, the Afghan government said, and six soldiers were missing after the militants' most deadly assault on the security forces in months.
Also on Sunday, in a possible blow to U.S. efforts to foster peace talks to end the Afghan conflict, the Taliban said they had suspended efforts to arrange a possible exchange of Taliban and U.S. prisoners due to the "complexity" of the situation.
It was not immediately clear whether the attack in the eastern province of Kunar was related to the suspension of talks on a prisoner swap.
In response to the killings in Kunar, a mountainous region bordering Pakistan that has long been a stronghold of the Taliban, al Qaeda, and other militants, President Hamid Karzai put off a trip to Sri Lanka.
"The Afghan president is saddened by this tragic incident, and therefore he postponed today's official trip to Sri Lanka," Karzai's office said in a statement.
Government officials said 21 soldiers were killed and three were wounded in the attack on an army checkpoint in Kunar's Ghaziabad district. Six remained missing, they said.
The government sent reinforcements to the area where the pre-dawn attack took place, Afghan Defence Ministry spokesman Zahir Azimi said in a statement.
The Taliban appeared to have been waiting for them. Azimi said the reinforcements "came under enemy attack, and a suicide bomber detonated his explosives near them". The suicide bomber did not kill any Afghan soldiers, Azimi said.
In a possible reference to al Qaeda or other militants who might not be part of the Afghan Taliban, Azimi also told Reuters that "foreign fighters" had taken part in the attack.
The Afghan Taliban, in a statement emailed to media, claimed responsibility for the attack. Local officials in Kunar said three insurgents were killed.
Sunday's assault was the worst since last September, when the Taliban attacked a convoy of Afghan forces in relatively peaceful northern Badakhshan province, killing at least 18.
The attack took place as U.S. and NATO forces withdraw from Afghanistan ahead a year-end deadline, shifting the bulk of the fight against Taliban and other militants to Afghans.
While Afghanistan's police and army are seen as having made big strides in their capabilities, doubts remain about whether they can keep the Taliban at bay, especially in remote areas like Ghaziabad.
It remains unclear if the United States and allied nations will keep a small force in Afghanistan after 2014 to support Afghan forces and go after al Qaeda, due to Karzai's refusal so far to sign a pact authorising a future troop presence.
Karzai urged neighbouring Pakistan, where Afghan and U.S. officials say Taliban and other militants are able to resupply and plot attacks, to help it fight militants.
"The President once again calls on the government of Pakistan to earnestly and sincerely cooperate with a strong will with Afghanistan and to take serious and effective measures in eliminating the terrorist sanctuaries that have continued to pose a grave and serious threat to both the countries," his office said in its statement.
PEACE TALKS IN DOUBT
Taliban spokesman Zabihullah Mujahid announced that the group's reclusive leadership, believed to be based in Pakistan, had suspended attempts to arrange an exchange of senior Taliban locked up for years in a U.S. prison for a U.S. soldier in militant custody.
In an effort to foster peace talks, U.S. officials recently took steps they hoped might revive U.S. discussions with Taliban representatives about the proposed transfer of up to five Taliban detainees out of Guantanamo Bay prison in exchange for the return of Bowe Bergdahl, a U.S. soldier who went missing in Afghanistan in 2009 and is believed to be held by Taliban-linked militants in northwest Pakistan.
"Due to the political complexity of the current situation in the country, the leadership of the Islamic Emirate has decided to suspend the issue for some time," Mujahid said, using the name the Taliban gave their 1996-2001 government.
"Therefore, the prisoner exchange process has been delayed until further notice," Mujahid said. He provided no further detail on why the decision had been made.
U.S. officials have long hoped such an exchange would not only free the only U.S. soldier to remain missing in the Afghanistan war, but would also be a step toward launching more substantive peace talks.
Robert Hilton, a spokesman for the U.S. embassy in Kabul, declined to discuss specifics.
"As we have said before, Sergeant Bergdahl has been gone far too long and we continue to call for his immediate release," Hilton said.
The Taliban statement comes as Karzai's government seeks to kindle direct talks of its own with Taliban officials.
Afghanistan's High Peace Council, a government body established to seek a political end to the war, said in a statement late on Saturday that a delegation led by Karzai aide Mohammad Masoom Stanekzai had met four senior "Taliban representatives" in Dubai last week.
The Taliban leadership has denied it authorised any of its members to participate in recent Dubai discussions and, in a statement last week, reiterated its opposition to holding talks with Karzai's government, which it deems illegitimate. (Additional reporting by Mirwais Harooni, Hamid Shalizi and Missy Ryan in Kabul; Writing by Missy Ryan; Editing by Robert Birsel and Robin Pomeroy)
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