* Heavy clashes in southern Afghanistan
* Taliban spokesman pours cold water on peace talks
* Death of leader Mullah Omar likely to spark succession tussle (Adds Pakistani comment on reports Omar died in Karachi)
By Kay Johnson
KABUL, July 30 (Reuters) - The Taliban's official spokesman disavowed peace talks with the Afghan government on Thursday, throwing fledgling efforts to negotiate an end to 14 years of war into disarray.
The statement came a day after the Afghan government said that Mullah Omar, the elusive supreme leader of the Islamist militant movement, had died two years ago in neighbouring Pakistan.
News of Omar's demise is likely to intensify a struggle within the deeply divided group to succeed him, clouding chances of a peace process that had already run into trouble.
In a reminder of the threat posed by insurgents stepping up their campaign to overthrow the Western-backed government, the Taliban captured a district in the southern province of Helmand that foreign troops struggled to secure for years.
The Taliban has taken control of pockets of territory across the country since NATO withdrew most of its forces at the end of 2014, leaving the Afghan army and police to quell the violence. Thousands of people are killed each year.
"We have heard from the news media that the second round of talks between the Islamic Emirate and the Kabul administration will start soon in Pakistan or China," said Taliban spokesman Zabihullah Mujahid.
"The Islamic Emirate has handed all authorisation to its political office and they are not aware of this process," he added in a statement that did not refer to Omar. The Taliban has yet to comment officially on his death.
Afghan and Pakistani officials had said that a second round of meetings would be held between Taliban representatives and the Kabul government this week. The two sides met for inaugural negotiations earlier this month in Pakistan.
So far, neither the Taliban nor Pakistani officials have confirmed Afghanistan's declaration on Wednesday that Mullah Omar died two years ago in a hospital in the Pakistani city of Karachi.
"We are aware of the reports and trying to ascertain the details," Pakistani foreign ministry spokesman Qazi Khalilullah said.
Afghan President Ashraf Ghani is keen to pursue the peace process, and has the backing of Pakistan and China, but the Taliban leadership is divided over whether to take part.
After the initial round of talks, a statement made in Mullah Omar's name appeared to endorse the negotiations as legal under Islamic law.
Opposing views on the peace process are tied in closely with a power struggle over who will be the new leader of the hardline Islamist movement that Omar founded.
A senior Afghan Taliban commander based in neighbouring Pakistan said the leadership of the movement was "at a crossroads", and resolving the succession issue may take time.
He added that a faction within the Taliban wanted Omar's son Yaqoob to take over, while another favoured the promotion of political leader Akhtar Mohammad Mansour, who has been among those who support peace talks.
On the battlefield, Helmand officials said the Taliban had wrested control of the Now Zad district on Wednesday after two days of fighting.
"Right now our security forces are still on the outskirts of the district and fighting with the Taliban," said provincial police chief spokesman Obaidullah Obaid.
Obaid declined to comment on casualties, but residents of the area, speaking to Reuters by telephone, said bodies of security personnel and Taliban fighters were lying in the streets after the battle.
The Taliban confirmed the capture of the district centre, saying weapons and ammunition had been seized.
Helmand has been a Taliban stronghold and centre of opium production for years.
British and U.S. troops began a concerted effort to secure the province in 2006, and some of the heaviest fighting of the war took place over subsequent years in small towns like Now Zad, most of them in the fertile Helmand river valley. (Additional reporting by Abdul Malik in Lashkar Gah and Mirwais Harooni and Jessica Donati in Kabul; Editing by Mike Collett-White)
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