* Agreement reached on vital draft of security pact
* Afghan elders frustrated by last-minute negotiations
* Washington rejects notion of "apology letter" (Recasts with draft agreement, Kerry remarks)
By Hamid Shalizi and Arshad Mohammed
KABUL/WASHINGTON, Nov 20 (Reuters) - The United States and Afghanistan on Wednesday reached a draft agreement on a crucial security pact, a day before thousands of Afghan elders are set to debate whether to allow U.S. troops to stay in the country after 2014.
Without the accord, the United States has warned it could withdraw its troops by the end of next year and leave Afghan forces to fight a Taliban-led insurgency without their help.
Thousands of Afghan dignitaries and elders are due to convene in a giant tent in the capital Kabul on Thursday to debate the fate of U.S. forces after a 2014 drawdown of a multinational NATO force.
"We have reached an agreement as to the final language of the bilateral security agreement that will be placed before the Loya Jirga tomorrow," Kerry told reporters.
Intense negotiations between Kabul and Washington have provoked frustration among the Afghan tribal and political elders who made perilous journeys from all over the country to the capital Kabul for a grand assembly to debate the pact.
Efforts to finalise the pact stalled on Tuesday amid disagreement over whether U.S. President Barack Obama had agreed to issue a letter acknowledging mistakes made during the 12-year Afghan war.
Kerry denied there had been any discussion about the possibility of a U.S. apology to Afghanistan for U.S. mistakes or Afghan civilian casualties during the 12-year U.S. military presence in Afghanistan. Such an apology would draw widespread anger in the United States.
"The important thing for people to understand is there has never been a discussion of or the word 'apology' used in our discussions whatsoever," Kerry said, adding that Afghan President Hamid Karzai had also not asked for an apology.
State Department spokeswoman Jen Psaki said the possibility of a letter, or some other kind of correspondence, would seek to reassure the Loya Jirga of the importance of the U.S.-Afghan relationship and to address concerns over civilian casualties.
The Afghan government said it had received assurances that an Obama letter would be provided this week to the grand council of Afghan elders, known as a Loya Jirga.
But Susan Rice, Obama's national security adviser, insisted on Tuesday that an apology was "not on the table."
The drawdown of Western troops has allowed tentative peace overtures between Kabul and the Taliban to gather pace, and Afghan officials arrived in Pakistan on Wednesday to initiate talks.
The Taliban have nonetheless condemned the Loya Jirga as a farce, and security has been tight in Kabul following a suicide bomb attack near the assembly ground over the weekend.
Insurgents fired two rockets at the tent where the last Loya Jirga was last held in 2011, but missed the delegates.
If the two sides cannot agree on a pact, Karzai has suggested submitting different versions of the document for the Loya Jirga to decide on. That caused confusion among Jirga members.
Khan Ali Rotman, who runs a Kabul youth organisation, said if the pact was not in Afghanistan's national interests, "we will raise our voice and not vote for it".
But a Kabul senator, Khan Mohammad Belaghi, said Afghanistan had no choice but to sign:
"We have to have a partnership with a country like the United States and we will vote in favour of it because it can protect us from threats from neighbouring countries, especially Pakistan, and the Taliban."
Violence spiralled on the eve of the meeting, with the Taliban attacking two high-ranking police officials.
Gunmen ambushed and killed the police chief of Marja district in the southern province of Helmand on his way to work, said Omar Zwak, a spokesman for the provincial governor.
Also in the south, guards shot dead a suicide bomber trying to force his way inside the house of the Kandahar provincial police chief, said Hamid Zia Durrani, a spokesman for the police. Later a bomb exploded at a hotel a few doors away, killing three and wounding 14, he said. (Additional reporting by Mirwais Harooni, Katharine Houreld in Kabul and Sarwar Amani in Kandahar, and Dylan Welch in Islamabad; writing by Maria Golovnina and Lesley Wroughton; editing by Ralph Boulton and Jackie Frank)